i read somewhere (maybe it was in this thread, too lazy to check
) that some guy keeps a ton of mollys with his rbp, and the mollys breed faster than the rbps can eat them
i'm getting a rbp tank in a few months, and i'm densely planting about 1/3 of the tank, so i think the smaller fish should be ok for a while, especially as the rbps become older, bigger & lazier
i'm guessing that neons, a few corys and maybe a group of small predatory characins (can't remember the name) could coexist with the piranhas as well, plus apple snails, some shrimp and a crayfish
of course not all at once in the same tank, but i think it's just a matter of giving the smaller fish lots of shelter and you can keep quite a few animals together with the rpbs, especially small/fast/nocturnal ones
plant a part of the aquarium very densely, put a big piece of driftwood in the open area and throw some floating plants in there
the small fish can't be that stupid and swim right up to the piranhas, can they?