Snails will come with plants. They're a pest. Get rid of them b4 they get to plague proportions. (Give them the good squish.)

As for your 6 fry, maybe get a 'fry saver' and put them in there to grow a little more. Once they're a bit bigger, they can be released with the other fish. Growth rate depends on a lot of other factors like tank size, frequency of water changes, water parameters and of course quality of food. Generally it can take a molly/platty up to 6months to reach full size.

All mollies, platties and swordtails will interbreed. Any colour will breed with any colour and u'll end up with weird looking babies. Advance lifebearer keepers will tell you that u can only keep one strain of any lifebearer a tank if you're going to breed them. (Mix up baby fish aren't worth anything.)