One: how often you change his water depends on how big the tank is. Ideally, you should be changing a small portion quite regularly, but if it's something not big enough to have a filter, you should do a water change no less than once a week. I usually end up doing more than one a week, and I also use a straw to sort of suction up poo from the gravel (you know, put one finger over the end of the straw and lower it into the water, then let go so the water gets sucked up...you must have done it with soda at some point in your life! Maybe I'm just screwy though). You could probably just use regular old cold tap water, but let it sit out for a day or two so it "ages."
Two: Fin rot is generally bacterial, but I've heard of fin rot fungus too, though I've never seen it. You will know if you have fin rot, and if it's fungus, I think the fins are pretty much gone overnight. Don't treat for fin rot unless you are sure you have it though, and even then, good clean water is the best thing you can do.
Three: it is possible your betta will bother the snail, and maybe even kill it, although I have heard of bettas getting along with snails often. It depends on the size of the snail and the temperment of your betta.
Four: pellets are really the best thing to feed him, as they provide the nutritional things a betta needs. You might want to consider offering freeze dried blood worms as a treat instead of a meal, but it might take a long time before your fish will eat pellets once he's used to the blood worms. Picky little buggers they are!
Good luck with your fish, you sound like you'll make a good fishy mom <img src="/images/graemlins/wink.gif" alt="" />
