I had a 55 gal tank for years, and had guppies in it with other tropicals. Mine did ok. There are some good articles on the net for caring for guppies, the type of water they like (temperature, ph, etc...), what food to feed. The more variety in food, the better they do. (not just flaked food- they also need worms and other stuff)
The fancy guppies they have now are very pretty to look at in a tank, and yes, the rule for a gallon per inch of adult fish is the rule. So when you buy them, and they are small, they must have growing room. I would go with a 20 gallon tank- by the time you put gravel, rocks and decorations in there, that is going to take up some of the room and make a little less water in there.
Absolutely cycle your tank before adding fish. There are a couple of good products on the market that will help you do that too. And I recommend getting a fish to help with the tank algae that stays small. Forget the name of it but they dont keep growing like those plecostomus. They stay about 4 to 5". That is going to take up some of the gallons in your tank too.
I dont recommend live plants- unless you rinse them THOROUGHLY. You may end up with snail problems. Those danged things can take over your tank!
Good luck- I enjoyed my guppies and though I sold my big tank- have recently considered a small one with some pretty male guppies. I have no interest in raising babies so dont want any females.