This is probably the most asked question about piranhas, hence a sticky thread had been made about it.
Piranhas kill things. EVERYTHING is a potential food source for them. If they are hungry enough, they will eat it, even ( more often than not) cannibalism. You can see others have tried to keep fish with their piranhas. And they pretty much all end the same way. So if you value the fish you are thinking about throwing into a piranha tank, then I would suggest absolutely no tank mates.
If you are prepared for the results then I say knock yourself out and try a cohab. I still have 2 convicts living with my Sanchezi which were supposed to be feeders...they've been in there over 6 months.
Whatever you choose, good luck.