It's going to break your heart and your pocketbook when your guinea pig ends up sick in the end since they come from breeding mills in poor conditions. I'd read up on mites, URI, fungus and learn how to sex them as well as pregnancy since that's what most Petco buyers end up dealing with.
Petstore, Breeding, Care and Housing Disaster Threads - Guinea Pig Cages Forum
If you can only get one (and from a pet store), you shouldn't get any. They do best in pairs.