Any small ray should fair well with discus....any adult ray will eat them . if u dont mind losing $50+ discus go for it. I keep cheap dithers with my rays because i end up finding them headless from time to time. Or very large fish that the rays cant pin and eat.
Any black Ray will get to big to keep with discus , I keep a mini marble with success with discus but they stay under 10" (male ) for any chance of success u would have to keep males and of species mini marble retic or hystrix