I have a coral beauty in my IM40, which is just a little bigger than what you plan to have. It seems to be comfortable in this for now, but I have two larger tanks with light fish loads if it grows and/or becomes stressed by the space. I have an arch aquascape, and that seems to give it plenty of live rock on which to graze and some good hiding spaces. It's currently sharing the tank with a filefish, flametail blenny, royal gramma, and a pygmy geometric "hawkfish" (perchlet). There's no aggression between them as they all have different tank habitats.
I'd actually recommend the IM40. While it's not my favourite AIO in the world -- the "sump" is weird -- is got a good mix of basic features and IM's random flow generators are a great (and cheap) add-on for the system. I bought mine used for $350 CAD.
I'm wouldn't recommend any other angels for a volume this small on a permanent basis. All of my other dwarfs have far more active swimming patterns and enjoy swimming the full length of their respective tanks. Perhaps a small captive-bred lemonpeel could work, but I've heard that even cherubs require 55g+.
