Advice on new kitty not getting along with the current feline tenants

Hello all.

I have two cats, Barbara Streispaw (12) and Puma Thurman (6), both of which I’ve had since they were kittens. A little background on them: they’re unfortunately not exactly friends, but they haven’t gotten in any fights in many years and tolerate each other pretty well. They’ll be in the same bed together, but they won’t be snuggling or anything like that, though we give them each lots of attention and both are very happy.

Three weeks ago, a family member was looking to get rid of her cat, and I knew she hadn’t been treating him very well (no physical neglect or abuse, just a lot of yelling), so I eagerly agreed to take him in. He is a bit of a “problem cat”, e.g. he likes knocking everything over, tearing things down, meows loud and often, and gets overstimulated very easily and is prone to biting (he never break skin, though).

For the first week, we kept him in my room so he could get used to the space, and there’s almost always someone home so he was rarely alone for long. When we first started introducing Charlie to the other cats, they were wary at first of course, and for a while they seemed to be getting more used to each other. However, over about the past week and a half, him and Barb have constantly been getting into fights. We never let them around each other without supervision, but when we do, they always start going after each other (usually Charlie initiates, but many times it’s Barb). If he attacks we usually shut him in my room with one of us, but even then they’re growling and hissing at each other under the door.

Puma doesn’t try to attach him and Charlie generally leaves her alone, but she will go after him to protect Barb (which is so sweet, and also a little funny because Puma’s less than half Charlie’s size and yet he’s still terrified of her.)

We keep him in my room with an automatic toy he likes whenever nobody’s home and he seems to do alright, and I keep the door shut in my room so none of the cats feel uneasy while they’re trying to sleep. I’m still trying to make sure to give them all equal attention, and none of them seem reticent toward me. I’ve introduced cats before, but it’s never been this bad and taken this long.

Any advice is greatly appreciated!