Some pet shops are very reliable and have a good knowledge of guinea pigs, although unfortunately it seems that many pet shops currently don't have the knowledge they should. Therefore it's a good idea for you to be clued up before you make a purchase.
Decline offers to sell you a rabbit companion
Many people have had rabbits and guinea pigs happily living together for years. However, there are particular physical risks to the guinea pigs, so new rabbit and guinea pig pairings are discouraged.
Rabbits and guinea pigs have different dietary requirements and therefore require different dried food. Dried guinea pig feed contains Vitamin C, but dried rabbit feed does not. Guinea pigs require Vitamin C in their diet. I have also heard rumours that rabbit feed manufacturers add rabbit antibiotics to the feed which can be harmful to guinea pigs.
Guinea pigs are tube shaped and low to the ground. This means every important part of their anatomy is vulnerable. A kick from an excited rabbit can cause injury and because of their shape, a kick to any part of the guinea pig can be serious.
For both rabbits and guinea pigs, the company of their own species is far better than the company of another. I know little about rabbits, but guinea pigs are certainly very social and the company of another guinea pig is almost a requirement for most.