Mollies actually do best in situation where they can move from freshwater to brackish water freely, but seeing as this cannot happen in an aquarium.... lol I am totally up in the air on which they prefer. There are great sites supporting both brackish and freshwater theories.
Personally, I would put as many female Bettas as I could stuff in a 30g tank. I have 6 in my 15g and plan for more, in a 30g I would put much more. Although, I have local breeders who support the "more females the better" theory.
Either way, good luck! Let us know what you decide on!