you cant keep any fish with a piranha permanently.
i had a few when they were under 1'', and now their 3''.
today, i came home to find a dead peacock bass, and a few dead tiger barbs. im not risking my more expensive fish, so i moved them out to my 60 gallon. (datnoid, jardini, african tiger fish,)
In my opinion your best bet for tankmates would be Black Skirt Tetras,or Exodons.But to tell you the truth i have had much better luck with the Black Skirt Tetras.
nothing makes a safe tankmate with piranhas its just a matter of time before they feel like eating the tank mates. sometime even eating other p's in the tank.