I have had a Koran Angel, I guess those and Emperor Angels are fairly similiar.
He was in a 55 gallon tank, he was sort of purchased by obligation, after I realized that the tank was too small! He was about 1.5-2'' when I got him, but he grew to around 4'' before his death (along with the rest of the tank) of ammonia toxicity.
He was in with several other small Damslefish, Clownfish, and a Herald's Angel. He got along very well with these, and several invertebrates as well as a small Moray Eel. He ate very well, preferring planktonic organisms to flake foods, and was always very heathy and beautiful.
Some people say that Koran Angels are ugly; I guess beauty is in the eye of the beholder (either that, or the people that said that were a bunch of shallow snobs

). I find the adult
Pomacanthus semicircularis has a subtle attractiveness that is clearly lost on the Emperor, with it's dazzling array of contrasting pigment.
All
Pomacanthus Angels make great aquarium choices, as long as a large aquarium is their home ultimately (150 gallons plus, once the adult size is reached) . They feed primarily on algae, sponges, and tunicates in the wild; in captivity, if they are not fed well enough, they may turn to nipping Corals and Clams. LPS seem to be the most likely targets; while SPS and noxious soft corals seem to be immune from the Angels' attacks.
-Lynden