Just looking for any tips or insight here, my partner and I have four cats (more details shortly) and multiple different issues for their socialization. All the cats were fixed at a young age and all have always been indoor-only. There are no other pets or people in the household.
The story starts when I got my first cat, T (I'm just going to use initials) right when I moved out on my own. He was a pretty typical cat, sometimes playful, sometimes lazy. He's also big, nearly 20 pounds. He's nearly 11 years old now, though, and while he has no problems moving around, he prefers mostly to sit and plays very little. Fine by me.
Meanwhile, my partner was on her own, obtaining her own cats. You have G (the oldest at 5 years old, prissy, but also the meanest when playing), M (an extremely friendly and playful boy, 4 years old, behaves like a dog a lot of the time), and Z (the baby at 3 years old, smallest of the group, least playful and most sensitive to new people). They have a pretty good working relationship, which is that G is the dominant one, Z is her frenemy, and M (the only boy in the group) stirs up trouble (but ultimately would never hurt either of them).
The problems started when we moved in together about a year and a half ago. We followed all the instructions about keeping the cats separate for a while until they all learned about each others scents, and then we tried slowly introducing them. Things initially seemed ok, but then T (my cat, the biggest and the oldest) started being aggressive towards G and Z (especially Z, my guess because of her size and temperament). This was strange because he was never aggressive in pretty much any other situation, but as soon as he saw either of them he wanted nothing more than to chase them. He was ok with M for a while, but then M started turning the tables on T - now M likes to bully and chase T around, despite the size difference. I think M is only playing, as he's very playful, but T doesn't want to play and he went most of his life without ever having other cats around to play with, so I don't know if he understands; it's also possible that M is defending his sisters from past aggression, even though they aren't around him at the time. T generally doesn't fight back, he just runs away and tries to hide unless he has no other options (which I think encourages M to chase more). I separate them as soon as I see it happening, but it keeps happening.
So now we're at the point of eternal separation - T is locked in a bedroom for half the day and my partner's cats are locked in a different bedroom for the other half of the day. Periodically I let M in to see T (because he cries at the door and some part of me hopes maybe it'll be different this time) but within a few minutes he's always attacking. And now G and Z get scared whenever they so much as see T, they'll run away and hiss at him, even when he's being held and they're completely safe. Things are worse now than when they first met.
And this isn't an awful arrangement, I'm honestly used to it these days because it's been like this for over a year, but I really do wish I wouldn't have to keep them like this for the rest of their lives (or let's be honest, the rest of T's life since he's the oldest) or else fear bodily harm. I just don't know if there's anything I can do or if the damage has been done.