I am dealing with almost the exact same thing as you. A 4 yo cat who is scared and timid and an 11 month old energetic kitten who wants to play, lunge and chase. We are at the baby gate stage (I put up a screen door).
Ok so, I have a 4 year old female semi-feral cat who was bullied by my male cat for 3 years. It hot so bad, all she did was hide and run from him. I had to rehome him. She settled down and about 6 mos later I got a female kitten. They hit it off GREAT!! My resident cat never showed so much...