start it out with a a few cheap decent size fish (its size or larger) and go from there...keep them well fed and they will remain ambush predators of smaller fish.
Actually from my understanding nothing can safely be kept with black piranha unless you have a school of them. Red bellies can be kept together from my own experience but all the info I have found black ones do not tolerate anything in their space.
I just got an orange hi fin wolf fish. So far he has not bothered any of my fish and the only thing small enough for him is a baby convict which he has not bothered. He sems to know that ruby reds are his meal tickets.