I think you probably could if they are all about the same size and the piranha is none aggressive. I trained my oscar, pacu, and chichild to be none aggressive. Just don't feed them any meat and they would probably get along. I would suggest to get the piranha small aligator gar small arowana a little bigger the oscar about a little smaller than the rest. They have to all be close to size though. Would also recomend a breeding shield to keep them seperate and wait until they are used to each other so they won't try to attack each other then remove the shield and see if they go together but keep a close watch.