I say no to ret terror and no to discus with rays! Here is why:
-- RTs are hyper aggressive and will quickly begin shrdding your rays
-- discus on teh other hand are too easily stressed and after sing rays "pounce" a few times will be afraid to come out to feed and stuff
If you want to keep cichlids with rays, try some severums, uarus, or just about any spcieces from the geophagus family