I agree that there's a misconception that your pleco is going to be a "janitor". Maybe on the first day if you have a ton of algae.
Either way, i have 8 RBPs with a small pleco the Ps have already outgrown. He's maybe 4 inches long and the largest P is about the same length. The pleco is out all throughout the day and does his own things. The Ps pretty much ignore him. Sometimes they steal his food, sometimes he takes it back from them. I've never noticed a bite mark or seen them messing with it. They say plecos are nocturnal but mine is out from the time i get home from work (5:30ish) and i'll see him out until about 8, and then he'll disappear again and he's out again but the time i turn off the TV for the night.