Many of the species mentioned come from the same waters the discus are from naturally, so many would do very well with them. Discus do not come from some lake of hot, super soft water where they are the only fish. They are from Amazonia, home to MANY fish. So there are a lot that can go with them.
Discus can do very well with: gold nugget pleco, bristlenose pleco, many tetras: (cardinal tetras, rummynose, bleeding heart, lemon, serpae, black phantom, and many others), many loaches (clown, angelicus, zebra, kuhili, yoyo, and others), marble and silver hatchets, angels, rams, and many other fish. Every fish is different so none of these are a guarantee, but all of these can work very well with discus. For some that are small like cardinals, they simply need to grow up with the discus. If the discus grow up with them they recognise the individuals and do not see them as a snack, unless you don't feed them for a while.
My discus get nothing but New Life Spectrum and are doing very well on it. You can check out pictures of them on my PhotoBucket page in my signature. Very meaty foods provide a lot of protein which if not balanced can be powerfeeding, leading to fast growth but an unhealthy and/or incorrectly proportioned animal. Fast growth does not necessarily equate to a healthy animal.
There are simply no stingrays that stay small enough to be kept in a 55.