I saw a special on the amazon that had a shot of a school of Ps tearing up a giant pleco. Looked like they were hollowing out a log. From that I'd have to say 'not a good mix'.
In my tank i have 6 p's. going on 2 yrs now. I introduced a pleco when i got them. then about 4 months ago i got another one.. a baby . Due to the fact that my now large pleco was lazy as heck.. it wouldn't do anything but clean the bottom of the tank. After i got the new one the larger one started doing its job.. maybe b/c it got jealous or fear of being replaced. I introduced another one b/c my cousin was moving. his life lasted 15 min until it was eaten. They have alway faught with the large pleco, he just goes crazy and knockes them around with his body. So i guess as long as your pleco defends himself. He should be fine
When keeping Piranha well fed, yes you can keep plecos of equal size with red bellies. I have tried it with Gold Spilo Piranha, Black Piranha and Piraya with no success!Always remember that ALL species of Piranha are unpredictable.