You could do a thorichthys species tank.
any of the thorichthys group would work.The only one I would not try in a tank your size is maculipinnis (formally ellioti) They tend to be more aggressive than the others.
Firemouths would work well.You could aim for a group of around 8.
Ideally you will want more females than males.
It will be very hard to sex them when juveniles but this is the is the best time to add your group while they are still young.
It may be worth buying a couple more than you actually want.As they mature you will be able to distinguish the sex easily.If you have more males than females you will be able to rehome the extra males.
I have found tanks like this just housing one group of the same species much more interesting in the long run.
Firemouths show incredible character and interesting behaviour when kept in groups of their own kind.
Try it,you won't be disappointed.