you can always say it depends on the fish, but here i would say don't mess with it. discus are too expensive and fragile to take a risk with. at the aquarium we keep angels and discus in a tank together, however, the tank is also much larger than your usual home aquarium, so space and territory are not really an issue. we also keep the oscars in a tank where they are the small fish, because they would likely eat/kill anything smaller than them.