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.