So there was an article on this in TFH this month by Jack Wattley in response to a reader's letter. The reader wanted to know if angels could go with discus and his response was:
"... for several reasons, I don't consider angelfish to be very good tankmates with discus. Many successful angelfish breeders maintain water temperature much too low for discus. Another point is that as angelfish mature they can become very aggressive towards discus if kept together"
So I understaned the water conditions optimal for angelfish don't necessarily match up with most discus perfectly, but agression? Most discus are semi as well? I would think proper tank size would help alleviate this problem. If this is such a problem with angels, wouldn't they be too agressive to keep with anything let alone discus?