I and ayone else with piranha experience STRONGLY advice absolutely no tank mates what so ever. The only thing that seems to survive with piranhas (not a guarantee but sometimes does well) are large plecos. If you dont want to risk your fishes life dont add any tankmates, its far too risky for comfort.
EDIT: Forgot to mention, even though there are some success stories, chances are that it wont work for you. Please research these fish extensively, they are not a fish for the inexperienced fish keeper. Im not saying that youre not capable of providing a suitable home, just that they are highly reccomended for people with experience. Perhaps a better substitute would be exodons? Or possibly a wimple piranha? (an excellent fish).