Intro issues - new cat is attacking resident cat

[cat nap]

My impression is that she stopped mostly because she was restrained, and she did have a puffed tail, but after a few seconds she settled down and went under the armchair and was fine as long as resident cat didn't move at all/didn't look her way.
In every other fight she was free and did chase and corner him, screaming and ripping fur off.

I'll try again with the leash and maybe I can get a video from my phone that would be much closer and higher definition.

New cat is absolutely adorable with people, never hissed at me for anything. When I get home from work I give her wet food and put the harness on while she eats. After a few days she's not bothered by it anymore. I leave it on for a couple of hours and do 1 or 2 shorts visits as long as everyone behaves.

I'm just really afraid this is not getting better, and she'll get more violent the more I try, while resident cat gets more stressed.

The thing with cat to cat introductions, though,...is that they are intended to be done in Steps...so that the "fight or flight" automatic response is lessened...and that gradually...over enough time...and exposure...the automatic response...of fear is also lessened.
It's actually supposed to be done over weeks, and months...and may take six to 12 months...like 'di and bob' mentioned in their above post. The slower it's done...the easier it is on the cats...since they have plenty of time to adjust to one another.

So like say...you make a Plan...and have the Stages of the cat intros...in place...but don't advance to the next stage...until both cats are feeling relaxed...kind of bored...and their fear responses are not there. They have become so accustomed to smelling, seeing, and observing each other...that the other cat...is no longer a 'strange cat'.
At the same time, too, they are each growing Confident...within the shared territory...and all of their resources are being met...territory, food, love and attention, Play,...expending energy, relaxation and sleep, etc.

What I think might have happened here, and it does a lot of the times...is that NC got to meet RC a bit too soon...and ended up in "battle mode"...."fight or flight"...and then felt she had no choice but to run RC off.
She was very afraid...and acted on pure survival instinct.

(the only thing that confuses me, ...is why they were able to sleep together on the couch...in the first place?
Unless they both felt very relaxed because their humans were there...and somehow...their "alarm system" of being afraid of the "stranger cat" ...did not really kick in.)

Accidental meetings always seem to happen. They are very difficult to avoid, since one cat always seems to want to push out, and explore...or challenge.
The only problem is...that when these accidental meetings do occur...and fights happen...that it takes a longer time for the cats to "forget"...or rather move back to a state...where they were not as 'high on alert'.
It's still possible, though.
It just needs more Time...to get back to a solid foundation...again.

Researching online I've never seen anything about cats being overprotective, could it maybe be some sort of jealousy instead? She is very vocal pretty much every time I'm not in her room or paying attention to her.

Yes, it could be jealousy.
Usually that is seen more in resident cats...but yeah...I suppose if the new cat bonded with the humans extremely fast...that it would make sense if she were jealous, too.
Let me see if I can find the thread...or article...where I previous saw where 'female cats have natural instincts for being protective over their territory...and their family', too. I didn't bookmark it...but I should be able to find it.

I never tried to put RC in the playpen, but I could give it a try and see how it reacts. The goal would be to get them used to be in close vicinity?

Yes, that would be the goal...but because RC is very sensitve...and you don't really have a time limit...then I would rather recommend that you do the Cat-to-Cat introductions ...at a glacial slow pace.
It would be less stressful on the cats.
(and the playpen...or cat carrier method...is usually only tried...when all the regular cat intro steps...are tried first.
It causes a lot of stress on the cats...because they are both confined...to a smaller space...and may not even help them...to reduce their ...automatic adrenaline responses. At least not in a slow and better for the cats' manner.)

There isn't a time limit to return her, but my reasoning is: If they are not compatible, the more I keep her the worse the trauma for her going back to the shelter, and the more attached we will all be. But IF she's happier as an only cat, I don't want to force her to coexist with him at any cost, it is very stressful for all. I'm just trying to understand what I'm dealing with, so I can make the right choice

I totally get that...and totally respect that.
You just want what's best for both cats.
And for everyone else involved, too.

It's hard...since I guess you really have to ask yourself...if you can create a Plan...and stick with the Steps.
It takes a lot of patience, Routine, Play...and effort, too.
But when you see small improvements...and have more time...then you'll probably see more Positive signs...similar to ones like you've already noticed...the less yowling/screaming.
If you think back on the fights and yowling...if in any way...they are different...then that is a plus.

Take some time to sleep on it.
And do only the gate Visuals for now.
Don't do any of the other things mentioned, yet,...not the harness meetings...or the playpen meetings.
This way, at least you will know if NC can lower her Fear and tension...behind the gate...and if your RC is willing to come up to the gate.

 

[trilobica]

(the only thing that confuses me, ...is why they were able to sleep together on the couch...in the first place?
Unless they both felt very relaxed because their humans were there...and somehow...their "alarm system" of being afraid of the "stranger cat" ...did not really kick in.)

Thank you for your time and encouragement!

After first week they were able to eat next to each other with the door open, no gates. At that point I decided to let them meet for a few minutes (or so I thought). They touched noses, he hissed, she moved away but they both settled down a few feet apart staring at each other, resident cat by the window, new cat on the couch with me in the middle. After a while they just fell asleep, and resident cat even moved on my lap and got back to sleep. Everything seemed great until I stupidly decided to get up and walk to the kitchen. Resident cat followed me as he always does to beg for treats, and at that moment, from nowhere, new cat chased him away to the other end of the apartment, screaming bloody murder, and cornered him. Here's pics from the first night, I was so happy it was going well.

After that, it's just weeks of a cycle where we go back to no contacts for a few days, things settle down, I try again to let them see/meet, and she'll just charge him at the first chance she gets (first and second time I let her out of playpen, then she jumped the baby gate, leaving her safe territory, to go chase him when I was distracted and he was sleeping).
And on the other end, resident cat is growing more scared of her and stressed, he doesn't eat close to the gate anymore, and he jumps on the counter to avoid her when he wants to drink water or greet me, etc. He always goes potty late at night after the last meal, while this morning I realized he was waiting for me to go near the litter box before he did his business. All of this is really distressing and not sure anymore it is worth it to make him miserable and her upset for months or even a year, just to force them to coexist if be friendly is out of the question.

 

[cat nap]

And on the other end, resident cat is growing more scared of her and stressed, he doesn't eat close to the gate anymore, and he jumps on the counter to avoid her when he wants to drink water or greet me, etc. He always goes potty late at night after the last meal, while this morning I realized he was waiting for me to go near the litter box before he did his business. All of this is really distressing and not sure anymore it is worth it to make him miserable and her upset for months or even a year, just to force them to coexist if be friendly is out of the question.

Hi T trilobica ...
I totally agree with you.
Especially if you see your RC is becoming so scared and stressed...that his behaviour has totally changed...and he is too scared to use his litterbox.

Sometimes NCs are not a great match.

I guess I was thinking that the recommendation...to give cat intros...at least 3 or 4 months...to see some results...or no results...would give you a better indication...of how things would be,...and how to move forward.
But you are absolutely right. They may never be friendly and just co-exist.
And would that be fair to your RC...to put him through all that stress.

As a last ditch effort,...could you reach out to the Shelter...and talk to them about all your concerns.
Ask them how NC was around other cats at the shelter? Did she get along with any of them?
Maybe she will need to be placed in an "only cat household"...if she is too dominant, shows frequent aggression towards other cats,...or even need some Prozac...like you mentioned before...in order to overcome her fear/aggression/stress, too.

I came across another member's post, back earlier in the year,...where they did say that they put their cat on Prozac...but it is entirely a different situation to yours...and I've never used it on a cat before...so would not even know how long it takes, when to use, ...and if it's even advised to use....when a cat is only showing aggression towards another cat....but not a human.
Anyhow, here's the thread... see Post#24...Kitten Introduction to a Sassy Adult Cat - Help!
And if you do a "advanced Search" ..you can bring up a few more threads, Search Results for Query: prozac for cat

If you do want to try this route, then ask your Vet about the transdermal prozac, that is rubbed on the ear, or the chewable form:
Fluoxetine Twist-a-Dose Transdermal Gel
Again, I don't know if it would actually help, here,...since NC is not being aggressive with you, ...just with RC.
And whose to say...how she would be...once you taper her off the meds.

Did the Nutricalm liquid help NC at all?
Members here, have mentioned using Composure, Zylkene, and gabapentin for various situations, too.
My main concern with medications being used in cats during cat intros...is side effects.
Medications for Cat Behavior Problems: Aggression, Anxiety, and More on MedicineNet

Thank you for your time and encouragement!

After first week they were able to eat next to each other with the door open, no gates. At that point I decided to let them meet for a few minutes (or so I thought). They touched noses, he hissed, she moved away but they both settled down a few feet apart staring at each other, resident cat by the window, new cat on the couch with me in the middle. After a while they just fell asleep, and resident cat even moved on my lap and got back to sleep. Everything seemed great until I stupidly decided to get up and walk to the kitchen. Resident cat followed me as he always does to beg for treats, and at that moment, from nowhere, new cat chased him away to the other end of the apartment, screaming bloody murder, and cornered him. Here's pics from the first night, I was so happy it was going well.

Thank you so much for the photos.

Yeah, they do look very relaxed...but if you notice in the second photo....NC does have her eye slightly open...so she is still watching RC very closely...even if sleeping with one eye open.
It could be like you said...jealousy...then...especially if she thought that RC had no right to be on top of your lap.

My thinking is that she was just not ready to...have a face-to-face meeting yet....but only because cats look like they are ready...but one week is too short a time to observe...all of the minor cat movements and body language that they exhibit.
Usually, it's recommended to take videos, for yourself,...on how they act around the gate...and then watch them back...slowly...looking for signals and signs that the cats do...with each other.

With Jackson Galaxy's method...doesn't he always say...Play, play, play...and have Food near the gate...to encourage Positive feelings.
Then he also talks about "Catification"...and having no 'dead ends' in the home...so that every cat can escape.
(I just don't know why he always seems to like to be bitten by the cats. I mean, if he's also a musician, then wouldn't those hands be very, very important. That's the only part I don't get when watching some of his shows on youtube.)

And hey, ...you did not "stupidly decide"...to get up and walk to the kitchen...
It's not like you have to remain there,...in one place...and not be allowed to get up and walk around your place,...go to grab something, commercial break, bathroom, etc.
Don't be too hard on yourself.
From what you described above...I think it's only natural...from them sleeping so close, with no growling or hissing, ...that any of us...would think they are doing fine.
Who knows...maybe NC did fall asleep...suddenly woke up...saw RC and freaked out...thinking he was suddenly in her territory.
Either way, she wanted to "fight" and not "flight"...by running him off.

After that, it's just weeks of a cycle where we go back to no contacts for a few days, things settle down, I try again to let them see/meet, and she'll just charge him at the first chance she gets (first and second time I let her out of playpen, then she jumped the baby gate, leaving her safe territory, to go chase him when I was distracted and he was sleeping).

I'm really sorry you are having such a rough time with this.
I wish I could be of more help.

Your playpen idea is also really good, since it protects the cat within...from being harmed.
That is why I had suggested trying to place RC there, and NC in a carrier,...but reading through your thread...it just sounds like RC is way, way too stressed...and NC is being too dominant.

Even your harness method is really good...but unfortunately, again, ...NC is not at a point of being calm.
I cannot think of any other ideas to help you out.
But do want to send you loads of good vibes, and good vibes for some extra calmness and peace between RC and NC...but realistically that doesn't help your situation.
Will send them anyways, though.