I have to give my opinion on guinea pigs - don't do it!
They are very sweet animals, but they are a LOT of work, they need very large cages, and they are extremely messy!!! Our GP was dearly loved, but he kicked bedding and worse out in about a seven foot arc on the floor around the cage. You have to be very careful about what and how much you feed them, you have to clip their toenails, and you have to clean their cages out almost daily. Guinea pigs take a serious commitment and a lot of attention.