Almost every cage sold for guinea pigs is too small, and one pig alone is its own problem

Two things the pet store gets wrong at the same time. Size first. The cages marketed for guinea pigs are mostly built to a hamster footprint and priced for an impulse buy. The figure used across rescue and veterinary guidance is around 7.5 square feet for a single pig and about 10.5 square feet for a pair, and honestly the pair number should be your floor even for one. Guinea pigs don't climb and don't use vertical space, so floor area is the only number that counts. Tubes and second levels are decoration. They popcorn and they sprint and they need a straight runway to do either. This is the rare pet welfare problem you can fix with a tape measure and one purchase. Second, they're herd animals, and a lone guinea pig is a stressed guinea pig. Switzerland went as far as making it illegal to keep just one, which tells you roughly how settled the question is. Two females, or a neutered male with a female, is the standard pairing. Two males can absolutely work but wants more space and a slower introduction. While we're in here, bedding. Skip cedar entirely and avoid raw pine shavings. The aromatic oils irritate their airways, and respiratory infection is one of the most common reasons a guinea pig ends up at a vet at all. Kiln dried pine, aspen, paper bedding, or fleece over an absorbent layer are all fine. Respiratory signs to know, since this is the thing most likely to actually kill one: crackly or clicking breathing, discharge from the nose or eyes, sneezing that doesn't let up, sitting hunched with the eyes half closed, not eating. Guinea pigs mask illness well and then fall off a cliff, so that's a call the vet today list, not a watch it for a week list.