Hey I have kept rays before, not yet with plecos. I think the safest way incudes the following:
-Large Sting Rays with small plecos
-Not any of the plecos that have been know to chew on wood and scratch acrylic (royals...etc)
-Large tank so in case pleco tries to latch on to the ray thier is more of a chance of it getting away...and more of a chance this action would be noticed.
-I don't know if all plecos originate in the same waters of the sting rays..however try to get them from the same habitat originated from.
-Constantly add veggie clips with zucchini/cucumber....to me if feels "slimy" maybe it would help the pleco to rather not watse calories chasing after a ray.
Thats all I could think of, good luck!