honestly, I think that a gulper piranha combo is bad. Either the piranhas will take chunks out of the gulpers soft flesh, or the gulper will get them as they sleep. Don't do it. As for neon tetras, expect to lose a few every once in a while, but otherwise, not too bad. Truthfully, pirahas just aren't really meant for tankmates (unless they are pygos, and the tankmates are other pygos)