Some species are supposed to be housed singly, and they ARE aggressive, but they aren't true piranhas. They're a close cousin of the pygocentrus species, called Serrasalmus. Rhombeus piranhas get large (13-15") will kill tankmates in close quarters, and have attacked their keepers. Occasionally a shoal of pygocentrus piranhas have attacked someone with their hands in the tanks, a toddler had to have her finger re-attached after her father cut it out of the P. nattererri's belly last summer.
But generally (and like bettas, there are exceptions to the rules) any pygocentrus species (nattererri, piraya, caribe) housed by itself will be shy, because they shoal for safety. But even solo piranhas will attack non-piranha tankmates, and that's never a pretty sight, and hard to clean.