tophat, I spend a lot of time just watching the fish to learn about them. At the store it isn't uncommon for me to go in, grab a book about something specific and then sit down on the floor and read the book and watch the tank for an hour or more at a time. At home, that is what I do for the first few months I have something "new". That's the one part of the hobby even most of the pros forget to do... you learn more from the fish themselves instead of the people who "think" they know it all. :P
As for feeder guppys, I'd stick to all male, you don't want any breeding, and the females are very plain without any color and get much larger. 3 - 4 males would be fine in that size of a tank, as the males stay very very small. The other nice thing about feeder guppys is that they are very sturdy. Don't mistake these for fancy guppys, which need the heated tank. Male fancy guppys are about double the size of male feeder guppys at full grown, too. Feeder guppys tend to be more tolerant of less than perfect water conditions, are a great beginner fish... they don't die so easily if you make mistakes until you learn everything you need to know for a perfect tank.
Apple snails grow to the size of softballs, quite a bit larger than any apple I've ever seen. Yes, I've seen them at this size... and they're also very dirty when they get big. The mystery snail is a relative of the apple snail, and they even share the same breeding habits. A full grown mystery snail is about the size of a golf ball, so yes, eventually the snail will become too big for the tank. I have never seen one adult size at less than 3 yrs old, so you'll have some time. Mystery snails are actually a tropical animal, and do much better with a heater in the tank. They can tolerate lower temps, but 68 is about their limit on the low end. Below that and they start to have problems.