I think that a group of 5 or 6 corydoras works well, perhaps a couple of dwarf cichlids as previously mentioned. I'm not sure if you can add any gourami.
so:
10-12 ba tetras
3-4 apistos (only one male) you chose the species
6 corys (you chose the species)
and if you are up for a challenge and have enough experience, 5 marble hatchet fish. (be warned as they are jumpers)