Actually, there are quite a few options. Some of the South American dwarf cichlids can do well in pairs or small harems (depending on species needs) in a 24" tank., as well as a select few rift lake cichlids in small harems. The ones that would work would be are between 2" and 3.5". For example:
a breeding pair of Laetacara dorsigera
a breeding pair of Mikrogeophagus ramirezi
a small harem (3 fish) of Haplochromis sp. "ruby"
a harem (5 fish) ofJulidochromis ornatus
Think of all the Apistos!
I, myself, am planning a 25g with a pair of Red Breasted Acara. But you're right, its all conjecture until we get water parameters. I love the planning stage, its so exciting!