It depends on the size of the tank, decor, the temperament of both fish, and last but not least, the species of P. However, it is not advisable to keep anything other than piranhas with piranhas. The ones that do live live a fin nipped and scared life. But of all the fish out there, common plecos are usually the ones suggested to keep with piranhas if you are set on keeping non-piranha tankmates. But I would suggest just going for piranha(s) in a tank, no other tank mates.
Ryan