I kept a common pleco with 5 rbp's for over 2 years. I love plecos and that's why I put one in with them. It would chase the piranhas away for food and everything. It would get fins nipped occasionally but nothing bad. I eventually got rid of the piranhas because I thought they were boring. Just make sure you feed them well. If you go away for a weekend and they get a little hungry, it could easily disappear.