I wouldn't suggest discus as there is many stories of discus picking at rays eyes and the stingrays disc. Not to mention an adult ray can turn the tables on a discus quite easy in the night! All depends on the personality and size of the fish. I wouldn't put discus in with my pups!!! If a fish is small enough the ray will eventually eat it or at least bite its head off.
with my 2 x leos, i keep 2 x ornate bichirs, 4 x severums
en-route is an 8" chilli red aro
had to give away my 4 x banded leporinus; nice lookin as they may be, they were nipping away at my leos