I had a school of seven neon tetras in my discus tank. At first they were doing really well. They added a nice burst of color to the tank when they would all school up. But then one died... and then another... then two completely vanished... then a another one died. So now I'm down to two neons. The two that disappeared are the one's that really concern(ed) me. A dead fish or two left undiscovered in a tank can cause a serious
ammonia spike and be fatal to discus. Fortunately, it never happened to my tank, but it is something to think about before putting small, frail, fish like Neon Tetras in a relatively large discus tank.
Also, I have four different types of discus in my tank (Pigeon Blood, Red Marlboro, Red Turquoise, and Ocean Green). They all get along just swimmingly (pun intended).