1. Will those fish get along? It really depends on several factors like the personality of your betta and how much plant cover you provide.
2. Will they be over-crowded in that tank? Cories are very active fish and IMO do better in a tank 20 gallons or more but with correct water changes you can make it work.
3. How old are the betta's they sell at petsmart? It varies, some of the smaller ones are probably 3 months old while the fully grown ones are probably closer to 6 months. There's no way to know for sure.
4. How do you perform a water change? You'll need to get a gravel siphon. Then all you do is unplug the heater (which you need) and the filter (which I suggest getting), start the siphon and remove 50% of the water. Use the end of the siphon to vacuum up uneated food and waste from the gravel.
You can do a youtube search to see videos on how to vacuum a tank.
Refill the tank with dechlorinated water that is roughly the same temperature as the water already in your tank.
5. How big will the mystery snail get? At most 2 in in diameter. IME the tend to max out at 1.5 inches when kept in tropical tanks.
6. Is there any certain breed or gender of betta that is best for a begginer? Not really. A short finned male or female will be eaiser in terms of avoiding fin rot or tail biting but as long as you keep up with water changes you should be able to keep any tail type you want.
7. If I get the tank set up on Boxing Day, when can I put the fishies in? I suggest looking into doing a fishless cycle. You can search the forum for more information but basically what you are doing is establishing a bacteria colony in your tank. These bacteria "eat" the chemicals let off from the fish's waste and convert it into a chemical that is not as toxic to them. Doing this allows you to only have to do partial water changes instead of full water changes.
The draw back of this is it takes about a month to do. You can do it with the fish in there but not with the cories or snail because they are too sensitive. I still highly recommend cycling the tank because it will make everything much easier on you.
However if you decide against cycling it is best to let the tank run for at least 24 hours prior to adding the fish, this allows the water temperature to become stable and for the decorations and everything to settle and remove any trapped air bubbles.
8. Which of the fish would be best to put in first? When you are ready to add fish always go from least aggressive to most aggressive. I'd put the snail in first, followed by the cories, and then the betta.
Look up how to properly acclimate the fish before adding them, this will keep them from going into shock.
Look at the sticky on bringing home your first betta in the Betta Care section. I think it will help you out a lot