I agree with BigJim. The problem with livebearers is that they have TONS of kiddos. You would definately need a heater for tropical fish since rapid changes in temperatures can kill your fish. I do think you should get rid of some of the snails. One or two would be sufficient to scavenge your tank. I noticed that you are changing your mind a whole lot. I really think you need to take your time deciding what exactly you want to stock in your tank. I don't mean to be rude, but when you put the fish in the tank, they are yours.
Here are some things I can suggest:
1. Have you decided if you are going to have a tropical (heated) or sub-tropical (unheated) tank. Deciding this will help narrow your options.
2. Do you want to breed the fish? Livebearers (guppies, mollies, platies, etc.) breed prolifically and you don't have room for all of the babies. Maybe look into something other than them.
3. Take your time. We are here to help you, so if you have any questions ask them. Just think hard before you make your decision.
If you do these things, the process will be a whole lot easier.