My experience with Discus is they are not hard to keep it is just very important you start with the right Discus. Anytime we have gotten discus from an online store or at the LFS they have had a rough few months due to various diseases. Now we get them from a local breeder and they are no trouble at all. Even though all the discus are healthy now, the locally bred ones just have more vigor than the others. They also breed a lot more often. Really a night and day difference between them. I think the angelfish/severum tank is nicer for that size tank though than Discus.
Anyway as for your stocking I would drop the silver dollars and the black skirts for a larger school of a prettier tetra. My personal choice would be bleeding heart tetras. Large enough not to be eaten by Severums, but not so large they disrupt your slower moving fish the way a silver dollar can. If you want two schools I also like the diamond tetra, and I find they compliment the bleeding hearts very well color wise.
You could also add a couple Acara(numerous species would work) or an eartheater of some kind. Most species of both typically hang around the bottom, and in your current set-up you are likely to really only have the cories on the bottom so more activity down there would look nice in my opinion.