u should have no prolblems whatsoever with a piranha in with your wolf. i had a dorado in with my wolf and my dorado beat the tar out of him and the wolf just sat there like a rock. not a very fun fish imo. but i dont think he will go after the p and i know the p wont go after him. if u have the room then a large pike would also be nice in that tank. but i would be more worried about a pike killing the p then the wolf