Is it heavily decorated or planted? If not, it shouldn't be much of a problem since those fish are pretty small. Cardex is right, mollies will reproduce pretty easily so you will want to watch out for pregnant mothers!
I've heard that for freshwater tanks, a good rule of thumb is that you should have about one gallon of liquid per inch of fish that you have. For example, you shouldn't have over 10 fish that are each 2 inches long in a 20 gallon tank. Obviously, this rule has exceptions but it may be helpful in later decisions.