I have very similar sized tank, 24"x15"24". I started off with a pair of keyholes, but ended up moving them as I felt the tank was to small length wise once they matured.
I currently have a pair of semi adult Laetacara dorsigerus and a male checkerboard cichlid in it. I think I would feel the same with the Laetacara as I did with the keyholes, in that once they mature the tank would seem a litle to cramped.
However I feel the checkerboard cichlid (Dicrossus filamentosus), or a smaller apistogramma species, would suite these dimension. Also becuase of the extra volume, you can add a few dithers. Dwarf corydoras, otocinclus, tetras and marbled hatchetfish*, possibly even cherry shrimp with the smaller cichlids like apistos and checkerboards.
*They grow much smaller than silver or spotted hatchetfish.