you SHOULD NOT mix piranhas with non-piranhas, over time. they will simply kill anything you put in with them eventually. you might have luck for a little while if the piranha is much smaller, BUT, eventually it will kill it's tankmates.
if you go for a smaller piranha, also, the wolf-fish might kill it.
basically there are two main genus' of piranha, pygocentrus and serrasalmus. you can more than 1 pygo per tank, 20-30g/piranha is the general rule. serras, with a few exceptions and if done by advanced aquariasts, can have only have 1 fish per tank, and, like i said, if remotely close to the size of other fish in the tank will kill it sooner than later.
some people have success for a while with exodons/buck tooth tetras in with piranhas, they are real nasty and aggressive little fish. the reason though is that they are VERY fast, and the piranha keeper keeps his piranha's belly full
good luck, there is ton of info on these fish if you look around.
pt