Which mollies are you wanting? Sailfins are a bit larger and more colorful than black or marble mollies. They can live in fresh, brackish, or full salt water, but the main concern is avoiding soft/acidic water. Hard/alkaline water is necessary for them, salt is optional, would likely kill your plants and biofilter.
Goldfish don't belong in a 20, they're really only suited to ponds. They're a cooler water fish, and grow waaay too large for a small tank like a 20.
Mollies are quite hardy if kept properly, and females will drop fry around every 3-4 weeks. Sailfin mollies are the same species, just different color morphs, black mollies are a short finned hybrid of the same species.