In addition to what's already been said, and as fishorama pointed out, discus can be intimidated & stressed by Angels as well as possibly the silver dollars, and fast-moving fish like your danios and sharks - and since they are social fish and do best when in a group of 6 or more fish, a lone discus may become a very lonely camper. One needs to do a lot of research on discus
traits & behaviors before getting some.
They're not hard to keep, but you have to know what kind of attention they need, i.e.size of tank, water conditions, what types of tank-mates they can compatibly be kept with, etc.