To my knowledge, platies and mollies cannot cross breed. (It may be possible though.)
Btw, it's not necessary to treat your tank with all of those chemicals unless there's a problem. The best preventative is to keep a clean, maintained aquarium, not fill it with chemicals.
Be careful about the guppies. They do breed fast, so keep an eye on the water parameters and look for deformed fry. I wouldn't recommend breeding in that small of a tank.
