Another thought: we as people sometimes forget about the 3 dimensionality of a cats world. You can make spaces high up that some can claim as their own. I know Dora likes being on top of the refrigerator. She is above the others and appears to feel safer that way. She can watch them without having to interact with them. A few high up escape spaces might help in your situation. The tree dwellers would be harder to get at and cannot be ambushed except from a direct approach. Up out of the way and hard to reach brings relative security and that reduces tension and the desire to fight with everybody.