I have a 20 gallon NPT community with 12 harlequin rasboras, 3 german blue rams, 1 powder blue dwarf gourami, and 2 otos. It is a really fun tank, probably my favorite. Every other month my harlequins spawn as do my german blue rams. Of course, all of the eggs are quickly gobbled up, but it is still a fun thing to see.
Whatever you decide to do I would get at least 5 more harlequins as they do much better in larger groups.
20g long NPT: 3 german blue rams, 1 powder blue dwarf gourami, 12 harlequin rasboras, and 2 otos
20g regular PT: 9 female bettas, 6 dwarf cories, and 2 mystery snails
10g divided PT: 1 male DT Halfmoon betta and 1 male Super Delta elephant ear
2.5g: 1 male Halfmoon betta