If the mollies are sailfin, then they get up to 6" and you're already overstocked. If they're regular mollies then they grow to 4" and you are about at you limit. Besides, you already got 6 fish in a tiny 15g. You sure you want to bump it up to 8 to make 1 fish for every 2 gallons? As always the choice and the consiquences are yours.