There are certain fish that can be kept with discus with no ill effects. In fact some of the species can really add to the look of a discus tank.
IE: Hatchet fish, cardinals lemon tetras, bleeding hearts, serpaes, black neons, cories, otos, headstanders, ancistrus, checkerboard cichlids, siamensis, ramirezi rams
All of these will thrive in the extra warm water that discus like and most importantly will not spook the discus and are all tame enough to allow the discus time to get to the food to eat properly.
Some of the following are HIGHLY not recommended for discus tanks for various reason such as aggression,(eating habits and bullying) overactivity, (fish that look like they've had too much coffee) inablity to survive for long periods in the high temperature water (84 to 90 recommended)
IE: black skirts, buneos aires, silver tips, angels, apistos, silver dollars black and red phantom tetras , flame and red eye tetras, large pencil fish.
These lists are from a recent TFH article about the pros and cons of discus.