First of all, what is the tank size you are planning on having? Mollies get quite big.
It all depends on the individual betta's personality. The rule of thumb is that anything that can be perceived as prettier or fancier than the betta (by the betta) then it probably won't do well in the tank. The betta often will perceive them as a threat and try to remove them, or vice versa. Some people have had good luck with harlequin rasboras and some other non-fancy fish. Generally, bottom feeders like cories, otocinclus catfish, snails and shrimp are the best tankmates.