The things you have to watch are feeding and aggression. Discus are slower eaters, more of a grazing fish, while healthy angels will gobble their food. This aggressive eating by the angels can sometimes stress the discus.
Angels seem to get aggressive at a younger age than discus, another thing that can stress the discus. The best way to keep them together is to get the discus, and let them settle in and get a little size to them. Introduce the younger angels after a quar period, this is one of the few times I would medicate a well fish. Discus are quite sensitive to internal protizoans & parasites, a round of metro followd by prazi would be a good idea for the angels before introducing them into the discus tank.