You can keep multiple female bettas, I would suggest keeping 1 or 4+ as they do create a pecking order, and dont normally fight, like the males. AlthoughI had a female who killed two other females and then died herself :'(
you might get away with maybe fully grown congo tetra, with a polyterus, but Im not sure its a risk worth taking.
you could try platydoras costatus, striped tlaking catfish,or such like.
But if you do get a poly's no cory's or danios, they would work out as expensive food sources.
synodontis catfish at a nice tank mate for them, and larger clownloaches, I keep both of these with my pair, along with an albino tiger barb, 1 female platy (who will have to be removed when the polys get bigger) and a L001 plec. Oh and two yoyo loaches, the tank they are in at the moment is fine for them, but it will need upgrading
if you go for a synodontis dont got for the small upsidedown cat as they will just get eaten, and prob cause you a few problems with the birchirs (polyterus' common name) as they are very spiny fish those USD cats