I had 6 RBP and 3 common plecos that grew up together in my 30, and eventually in my 125. In the end the 5 remaining piranha were about 7 inches and the two remaining original pleco were about 5 inches.
I added two ~5 inch pleco when the piranha were about 8 months old. I read somewhere to keep the lights off for a couple of days after introducing them, and it worked for me.
The key, aside from keeping everyone well fed, was having caves/areas for the pleco that were inaccessible to the RBP. I think it also obviously helped that they grew up together.
Also, the new pleco should not be smaller in length than the piranha.
IMO plecos are the best tank companions for RBP because they are the only fish that have armor AND serve a purpose as glass cleaners.