this is true you can keep piranhas with other fish, to an extent, while most commonly we know the end result, some htink its in humane trying to keep fish that get picked on fin/scale nipped or worse, although many people insist on trying it, plecos are hard bodied "armor" if you will but there eyes are not , tetras are commonly tried being fast and small, a good tetra kept with piranhas commonly is the paradoxus exodon, aka buck tooth tetra, also an aggressive specie, i have seen them even feed together and have many firends who keep them with caribe reds and more, never serrasalmus specie, but they do in fact lose some to the piranhas, armored cats i think fall into same category as plecos that i stated. Although it may be succesful to a point 9 times outta 10 ending result is in casuality, again caption for this thread is IMO so just my two thoughts, i have kept and sold piranhas for a long time now and i get this question very much, and always answer it the same way.