Bear in mind when buying they like company, they can become depressed when kept on ther own. I have two and their pains in the bum lol, they always squeal VERY loudly when they want feeding.
For substrate I use newspaper to line the bottom and then a generous layer of woodshavings over the top, and when you say 20g tall you mean a tank? A traditional wooden rabbit hutch would be better, but due to the fact its wood and soaks up fluids, a plastic indoor hutch, (they have a wire top and look kind of like oversized hamster cages) would be better still.
Mine have a small handful of the petshop bought cereal every day, but mainly eat fresh greens along with chopped carrot or apple, and things like cucumber and tomatoes make great treats!
Guineapigs make great pets and I think you'll love them! But try not to pick them up unless you have to (to move them or check their health) they don't like it, they love to be fed treats and tickled behind the ears though
