I just thought I would add that from my experience with plecos is that they are quite different form individual to individual and species to species. I have had some small species like a bushy nose that was the smallest fish in the tank yet was king of the tank with a monster appetite and I would not put it past him to suck on another fish. Yet I have had 15"+ regular plecos that were so dam lazy I would not worry about them to much. So I see how it works for some people but could also easily be a problem in some cases as well.
So in conclusion, I personally would not risk it with rays.
