The mollies sound good for now. Stick with 3 of then since it's good match (1 male 2 females). The tank will spike (not too high) in ammonia after a few days, because the mollies will be producing a new, constant source of it. The bacteria will soon catch up. After about 3 weeks, the water should level off and then you can add the cories and snails.
The 1 inch per gallon rule is not a good thing to think about. Say you have a 20 gallon tank, so you can have a 20 inch fish right? Well that fish won't have enough room to turn around even in a 20 gallon tank. It's more like, don't overstock, in a 20 gallon, nothing over 3-4 inches, and make sure they get along. The bigger the tank, the bigger the fish obviously.
Hope this helps, and welcome to AC.
Edit: Forgot to tell you about live plants. If you really want them, then fine. They can be great, but sometimes they can be a big hassle too. Some people here, (don't exactly know who) have fully planted tanks with CO2 systems pumping extra Co2 for the plants into the tanks. I personally don't think it's good for a beginner fishkeeper (not to condesend).