I've kept 6" motoros and flower rays with 4" geophagus with no problems. If you're looking to keep larger and more aggressive cichlids, it's a recipe for disaster. I had an instance where tails were nipped up past the barb overnight, ending up in the ray's demise. The nips started out as investigation, but ended up as, "hey...this could be food!"....so please be careful.
If you're talking about the smaller rays, they'll more than likely be smaller than the 6" motoros when you get them creating more of a risk. With larger rays, you have to make sure that the cichlids can't be pounced on and torn apart...It's a delicate balance I know, but some trial and error has to be done. Just monitor the situation or search the forums. There are plenty of threads indicating what people have kept successfully, as well as the horror stories.