Hi,
I had an adult emperor for a number of years. He was in a FO with a number of other fish. They seem to do best when they are the dominant fish in the tank, mine wasn't very nice but he wasn't overly nasty either. Despite what books may say, I have found them to be a very hardy fish. The LFS I got it from said the same, I thought I'd give it a go (as this is a very reputable LFS, this guy knows his fish as well as anyone I've ever come across), and it worked very well. However, I wouldn't put him in with a Koran as they will probably beat the crap out of each other and you will end up with one or the other dead. A pygmy angel is OK, I had mine with a C. bicolor and they got along fine (the bicolor killed all other Centropyge though).
Also, in aquariums their colour often do not change at all, so if you buy a Koran and an Emperor you will end up with 2 very similarly coloured fishes. Even if they start to change, they often will not colour up all the way to adult pattern.
Regards,
Tom