It's not too many fish for the tank (mollies are too active and get too big for a 20 gal imo
). I was talking more about the fact that it sounded like you added them all at once after cycling the tank.
Feeding every day might help as well. You can skip one day a week but I don't see a need to fast them every other day.
Do you have plants/hiding spots for the fish. Mollies and platys are livebearers and will breed like rabbits. Do you know the genders of your fish?