I have one too, it's amazing!
When I got it I allredy had a C.bisbinosus in the tank. The C.acanthops came at the same time with C.bicolor, and they all got along nicely...
Recently I added a Pomacanthus navarcus, and still no problem.
My tank is much bigger than yours, but the angels all hang together and haven't had any aggression towards each other.
I'm planning to add a C.potteri and C.loricula soon...
IMO if you want to add another angel, try to pick one that is as different in color as possible. C.bicolor might be a good choise?