Mollies do fine in brackish or fresh water. I wouldn't recommend keeping a betta with mollies in that size tank simply because mollies are manic and aggressive eaters that will sometimes fin nip slower moving fish with large, flowing fins. Corydoras, tetras such as rummynose, cardinal, ember, etc, harlequin rasboras, beckford pencilfish, kuhli loaches, otocinclus and many others would all be peaceful tankmates that could work. As already noted though just be sure to have a backup plan in case the betta is overly aggressive.