It won't work. Nothing will last long term with a large serra, they are solitary fish. If you cohab them with anything I'd recommend tiny dither fish that they *might* ignore. My manny would have torn into anything remotely close to him in size, no question.
if you care about the other fish then don't keep them with piranha.
even if other fish are big enough not to get eaten in couple bites it can still die from stress from constantly being pick and nip by the piranha.