I used to keep an oscar, 3 RBPs, a JD, and a spotted gar in the same tank. they got along fine, as long as they were well fed. piranha and oscars live together in the wild, and oscars aren't extinct.
that being said, I wouldn't try to keep more than 3 piranha with an oscar in a 55 for very long. they're gonna need a much larger tank before a year is over.
IF YOU SEE YOUR NON-PIRANHA'S FINS DISAPPEARING, YOU AREN'T FEEDING YOUR PIRANHA ENOUGH. CORRECT THAT SITUATION!
Overcrowding-Can be accomplished as well-You are certainly pushing the boundries with that many in a 55----But you can raise them to a fair size before they are going to have to be moved....Setting the tank up right is the key....
But you better have plans for selling most of them as they grow-Or buying a bigger tank or provideing them with a pond big enough to suite them all....
ive had success keeping p's with armored catfish, i currently have some rbp's and a hoplo cat and pleco in the same tank, but its not really a question of if they rbps will attack the catfish its a question of when but so far they have been fine for the past year in there but i plan on moving the hoplo out soon
That depends on the situation and the piranha. Many species of piranhas strictly feed on other fishe's scales and fins. Of course, in this situation, that would be true.