It really depends on the piranhas themselves for the most part it's a no but a fair amount of time plecos are successful tankmates even if nothing else works with that particular group of piranhas.
Get a pleco a fair bit larger (3-4") than the piranhas you have if you want a little higher chance of success. But still don't get too attached to your pleco you may find him missing (or half of him missing) one day.
well id say no since piranhas are aggressive there not maneaters by that i mean eat anything they see but they will go for raw meat..but anyways um id say it would be helpful if the pleco was about 5-6" so it was decent size but yea id grow it out to that size or more before i put in piranhas even though the pleco has armor for example common pleco..but yea they might beat him down till hes gone or so..so its a 50/50 chance your taking either way good luck
Plecos are probably more aggressive than piranha. As long as the RBP dont see it as food you will be fine. For example a 4in P isnot going to mess around with a 10in pleco. Not a chance. For the most part. If you keep your P's well fed a pleco can protect himself from only 3 piranha. You will know in the first couple hours if your P's percieve the pleco as food. Chances are they wont unless you feed alot of big feeders, then they can get aggressive and territorial. Most RBP's are scared little girls with teeth haha. A very shy fish.