I tried adding two tomato clowns to my tank with an existing tomato clown hoping that 2 of the 3 would pair up. Well, 2 of the 3 did pair up, and when they did they made short work of the straggler. I would think your oscellaris would kill an intruder percula. Oscellaris and percs or more doscile than other clowns, but a pair of anything can wreak havoc on just about anything. For example: a pair of 2-3" Convicts can dominate an aquarium and kill off all inhabitants regardless of species. My breeding pair I use to have in a 70 gallon killed other meaner and bigger cichlids. I wouldn't try it. If you were to buy a perc, false perc and lets say a naked oscellaris and/or black oscellaris all at the same time you may have some success.