Relationships among multiple indoor cats could devolve into a slugfest. The head banana may lurk in the shadows, pounce, and scare the snot out of an unsuspecting subordinate. Wounds and abscesses can be lanced and irrigated but the fear can last a lifetime.
Be observant. Watch for one cat blocking access to the litter box. Does he stare at the mild-mannered kitty until she gets up and moves? Is anybody getting slapped around while resting? How about a victim cat who skulks along the baseboards doing a terrified rodent imitation?
The rules of social engagement for crowded kitties are very different than for human beings. We can escape the drama of home life. But stuck inside with few natural choices the feline despot, who can’t even get out to go sport mousing, is coping the best he can. Guilty of nefarious acts indoors he might actually comport himself with civility if he were free-living, able to engage in innate survival behaviors. If your victim cat acts timid and helpless, Mr. or Ms. Macho will be unable to resist. Nobody is set up to succeed.
Give the gift of vertical real estate. Indoor cats need at least a couple of floor-to-ceiling carpet covered cat trees with platforms and hidey holes. Locate these perches near windows for live prey viewing. Bird feeders or bird houses stuck with suction cups to the outside of the windows will have your cats believing that they’re fishing in a stocked pond.
Privacy matters, especially for low ranking cats. Multiple hide boxes at various heights in different rooms will allow everybody to catch a break from each other. In the evening you can be the ringmaster with stalk-and-pounce toys like feathers on a stick. With more of their primal needs met your kitties may actually avoid each other. You can get the full list of Feline Environmental Enrichments at https://drjeffnichol.com/feline-environment-enrichment/
Were you hoping your cat scout troop would join paws and sing kumbaya around the campfire? Sorry about that. You can promote a more social emotional state for all cats great and small with Feliway Multi Cat diffusers. The antianxiety supplements Anxitane and Zylkene may also help. Most important, don’t add any more cats.