I am afraid that your tank is already fully stocked and then some. When the platy fry start to get near their full size, you will have a very heavily stocked tank. The mollies will grow to about 3 inches each if given adequate food and proper care. Mystery snail adults are figured at 2 gallons each by snail breeders and the platies will grow to at least 2 inches each given proper care. With no platies in the tank, your tank would be stocked to a level usually recommended for new fish keepers. When people get adequate experience and become quite adept at caring for their fish, the stocking that includes the platies would still be quite heavy. Please don't add any more fish to that tank.