for reference ive allways had at least one common pleco with my rays and have never had a problem.
i have however lost several plecos that were housed with rays for several reasons and lost quite a few because the rays ate or tried to eat the plecos.
i currently have a vampire pleco and common pleco with my rays but have had others with no problems.
i also belleive the suck marks that the plecos are blamed for are commonly caused by other rays rather sucking on the rays discs (as i did myself but found out different when the same marks appeared when the pleco had been removed ) rather than the plecos that get the blame because of the "sucker mouth" name ).
I've kept plecos with rays as long as I've kept rays. Only bad thing that ever happens is the bigger rays take a Pleco down or beat it to death. Never seen this soo called rumors of plecs trying to eat rays.