Killifish are usually fairly 'darty' swimmers, and generally quite colourful. They may prove a target for female bettas. However, there may be some native killie species available that are less colourful and a more suitable option.
Ideally, you want something peaceful, that isn't too much of an active or unpredictable swimmer, and that isn't going to be targeted for aggression by your bettas.
Rasboras and some tetra species are a good option. I personally like the look of some of the smaller species of rasbora a little better than the tetras.