Only some cories will be compatible as a lot will not stand the high temps discus require. You will need to research the exact species you are interested in.
Assuming your tank is 18" high I would keep 4 discus in it, with a bare bottom and very few tank mates if any. I would also warn that keeping 4 can produce problems with bullying. It is normally suggested to keep a group of 5 or 6 as a min. You should not keep 2 on their for long, you will likely find one getting bullied after a while. A general rule is 1 discus per 10 gallons, so you can see a larger tank would be better. It takes experience with discus to bend this rule slightly.
Don't want to sound rude, but if you need to ask if your tank size is ok and which fish can be mixed with discus your probably not ready to own them yet. You really need to research the discus more and decide if your ready for them. Do a search on here for Discus and you can learn practically everything you need to know.