if you do try this, their are a few things that you can do to reduce aggression.
*feed the piranhas before doing anything
*remove the piranhas from the tank, re-arange the rocks and plants and add all the piranhas together
*add some other "dispendable" fish like tetras, silver dollers, goldfish, guppys or anything really
*provide lots of hiding spaces, and cover with big bushy plants
Depending on the size of your tank to see if theres enough room for all to be compatible and not to disrupt the previous residents territorial grounds. Take Innes advice, something I would recommend also.