I have kept small plecs in my guppy tanks with no problems. While they eat any leftover food, most algae and any dead bodies, they do poop a lot and so the tank isn't actually any cleaner.
Plecos dont eat fish poop, they make it.
I was also given a 6" goldpot pleco that had learned to slurp up small guppies. But I suspect it was underfed.
A bristlenose pleco would probably be ideal, unless you want to use the feeder tank as a growout for a proper monster plec
Ian