I have a 6" sailfin pleco, a 4" common pleco and a 2" clown pleco in my redbelly tank. The first two are with my piranha's for almost a year, the clown pleco just two months... Only the big one gets nipped occasionally, but, amazingly, its fins are usually healed the next day, even if they were pretty torn up.....
I would say pleco's are the safest fish to keep with piranha's (perhaps besides raphael cats), but you will never know how old it will get: it might be killed within a day, or after living with your piranha's for years.....