you should really consider doing a more controlled introduction like the articles say. We went through the same thing - hissing, vomit, hiding, resident cat looking sad etc. We felt awful after the first week thinking we made a mistake and wondered if we would have to find a new home for the cat because we were ruining our resident cat's life. I'm sure it's traumatic for you and the cat, but it's been two weeks and our two kitties are awesome friends. We kept them separate and did scent swapping etc until they were both playful through a small crack in the door, then we slowly allowed them to be in the same room. I hear that if you rush them together it could ruin their chance at being friends