Not quite sure what you're asking. And you didn't specify what kind of African frogs---Dwarf or Clawed frogs? In any case, I wouldn't combine frogs with guppies. They have different temp/habitat needs. And if you want the guppies to breed, frogs have pretty hearty appetites and will eat just about anything. But an all-guppie tank sounds great. Bear in mind that they will mate, so make an informed decision on colour and male/female ratio. Go to a store where the employees know what they're talking about and ask plenty of questions. I haven't had guppies since I was a kid, but try Googling guppies and you're sure to find a helpful site that will give you lots of info.
However, if you really want frogs and have a 20 gallon tank. You could get two African Clawed Frogs (that would have to live by themselves---no fish) or you could get a couple of African Dwarf Frogs and you could definitely add some fish. Make sure you're very specific about what kind of frog you're looking for at the pet store. If the frogs are albino, they're clawed frogs---period. It's easy to tell the difference, once you see them both. Make sure you know what you're looking at before you go that route. But if you get ADFs, I wouldn't suggest gupppies, but maybe a school of tetras (any of the colours) or white clouds. Even cat glass fish (but don't do the painted ones---it's a cruel thing to do to inject the fish and they don't live very long anyway because of it). ACFs can live in harmony with any of the peaceful small fish. Do some research and see what appeals to you. There are so many varieties of tetras that you're sure to find something you like. Just be sure to get a school of whatever peaceful fish you put in there. That's at least 6-8 fish. They're happier that way and you can really see them in their natural behaviour (or as close to natural in a tank). But with 20 gallons, you can really have a nice little habitat. Good luck and let us know how it goes.