Personally, I used to have a 8 inch pleco with my 10 inch RBPs. They never bothered him. On the other hand, I have seen lots of dead pleco pictures over the years on this forum as well as Piranha-Fury.
I am not a piranha pearson but I know that plecos have armor on them... an ocasinal bite would be okay but they might be able to get though it's armor and if not flip it over and get its unarmored belly...
I have 3 rbp in a 55 gallon they are about 5 inch and they have been fine with a 6 inch common pleco for about 6 months now...just make sure you have a place for the pleco to hide during the day were the rbp cant get to him
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