Discus are slow eaters that are easily stressed, so avoid fast eaters that can eat the same size of food (I am asuming you feed small pellets) or fish overly active at night.
A lot of small tetra can't eat those pellets (or just slowly), so you should be able to pick whatever you like in the store. My personal favourites are Lemon and red Phantom Tetra.
If you are not a SA purist Guppys and Platys might be a colourful addition too.
I have seen a lot of people keeping them with Corys and Plecos, however as most of them enjoy a good current keeping them in a Discus Tank seems a bit unfair to me.
Just make sure whatever you add is free of disease.