It can depend on the species, coloration, size of the specimens,timing, as well as LUCK. Angels are my favorite so I've made efforts to try and keep more than a few. I've had success in the past keeping a Flame, Lemonpeel, Potter's and Flameback in the same tank (125g FOWLR) and currently have a Flame and Cherub in a 55g. Adding them together will certainly help if you decide to try as well as making sure that they can establish their own territories. Have a seperate "outcropping" for each fish. If you just have one contiguous run of LR (without any separation), the dominate fish will eventually make the entire run their territory. Different colors can also lessen aggression, as long as they are added together.