I had one 6" RBP and 4 oscars of the same size and he killed all of them. Don't know how he did it, but he did. The oscars always hid up in one corner while the piranha just sat in the middle of the tank.
Also a terrible idea. ANYTHING you put in a tank with RBPs besides other RBPs will more than likely almost always become dinner.
I had a pleco with mine for about 7 months, then one morning i woke up and found his head under a rock. The pleco had been in the tank with them since the begginning too.
it's just not a good idea to try and co-hab RBPs with any other fish (besides other members of the pygocentrus family of piranha)