It is all hit and miss Joe. Some plecos will suck on fish and some don't all depends on the individual pleco. I had a common in with my rays for 3 months with no problems. Then one day I cought him go suck on my catfish, took him out right that second, cause I didn't want him doing it to my rays.
Colin, (rvrrays) has a 12in common in one of his ray tanks.
I have seen it work, and not work, it is all just hit and miss. But it is just best not to have them, cause then you don't have to worry about it.
Andy