misconception. mollies these days are quite often kept completely fw, not bw anymore. mine are completely freshwater and healthy adn happy. it depends on how they are kept before you buy them (if the store keeps them fw, do that, if they keep them bw, do that as they will have a difficult time acclimating from one to the other). also, there are three types of mollies, regular which have a paddle shaped tail, balloon which are not as long and have a big ballooned belly, and lyretails which ahve lyre-shaped tail fins. if you get any of the first two, you may be okay, but if the mollies are lyretails, i wouldn't recommend it, the betta may mistaken the fancy finnage for other male bettas and attack. though bettas temperaments vary, and some will be fine with the regular mollies, other will attack anything in the tank (if you get a betta too, be ready to transfer it to its own little tank, they ahve some nice 1 and 2 gal at the shops). as far as a betta with cories, again, most likely they'd be okay, but you just never know and owuld have to watch them for a time.
in a tank that size, i really wouldn't recommend the mollies, not much room. get the betta its own tank.
