Just because you have seen it doesn't mean it is right...as you said you are 13 years old....which to me means you don't have a full knowledge of how things work....you shove a fish in a small place it looks fully mature but in reality you can't stunt the growth...this does not mean just in length of the outside...but it's internal organs as well.
you came here asking a question and everyone is trying to answer it and give you really good advice......please don't act like you can school everyone here when most of them have been doing this much longer than you have been alive. hear their advice and use the newly found information. yes a molly/platy can grow in a 1 gallon...BUT i have to stress it they need room to swim just like as a child you need room to run and play..otherwise you will end up fat lazy sitting on a couch and dying of organ failure from who knows what illness diabetes, liver failure, high cholesterol, kidney failure etc....
even with daily water changes it won't stop the stunted growth. like i mentioned before you should put that platy in the 5 gal maybe 1 or 2 guppies and call that good for now.
OH by the way platys mollies and guppies won't cross breed....where ever you got that info is wrong....otherwise i along with many others would have hybrids all over the place....different species can't breed, the genetic make up says so.