Go to applesnail.net - they have a lot of information on taking care of apple snails; best water parameters, appropriate tank mates, etc. They can be a lot more sensitive to things in the water than fish, and harder to keep healthy than most people realize - but worth it!
You need at least 2.5 gallons per snail. Bettas and angelfish will usually nip at them and keep them from eating. I'm not sure about mollies.
All of paytertot's questions are good ones.