I'd second the question - how big is your tank?
Whenever I've kept bettas in community-type set-ups, I've included either swordtails or platies without issue. I've also kept them with endler's but that was a one-off - I don't know how well it would work in other circumstances. Mollies or guppies would be a definite no-no, IMO, but I don't think it's that clear-cut with platies.