An angelfish would likely be a good addition imo, most are quite compatible with peaceful, acara/geo shaped cichlids ime. They can vary in size, according to original species, breeding strain, etc. Some I've had would fit nicely in a 40 and some would be big for a 40.
I say most would be compatible, they can vary in temperament-- yes,
that again.
Example is I've had two groups of wild Peru scalare, first group got along great with each other. Current group fusses with each other constantly, though quite peaceful with the geos I'm growing out with them.