If you have a realtivly lasrge tank and it is not over stocked and you keep plenty of decor and maintain and feed well then you can obtain this goal.
Works best when both are raised from a smaller size
I think it all depends on your piranhas, and not tank size or other things, however I do think that size of the fish is an important factor, as I have quite a large severum with my piranhas in a 20 gallon (yes quite a small tank) I only have 2 piranhas, but I also added some zebra danios yesterday, and I havn't seen the piranhas or the severum even take an interest in them.
But My piranhas are not used to eating fish, as I never feed them fish, and never did.
They once ate a pleco I added, but that is all.
I think it is just about how chilled are the piranhas, I guess mine are pretty chilled.
they do or will eat them especially if the size difference is big they will see them as food
keep cichlids in a cichlid tank and piranhas in a piranha tank best way but if you have to do it take note of what other people will tell you that do keep other fish with p's