What else would be in the tank? Agressive fin-nippers would harass the poor angels to death. Anything too small would become a snack.
As far as what to look for goes, it's really a matter of personal preference. They come in all colours with various fin lengths. Whatever you decide to get, make sure they are active and don't have crooked or bent fins.
They will eat just about anything. Angels are carnivorus. I feed mine a mix of flakes, freeze-dried blood worms, freeze-dried brine shrimp, and live food (guppy fry or tasty-looking bugs I catch in the yard).