Hi, I hope I'm not too late to stop you but NEVER NEVER NEVER should a rabbit be housed with a guinea pig. This trend was started by pet shops and it has become the norm for people to think its ok.
Rabbits have huge strong back legs which they are prone to lash out with, not always intentionally, sometimes they are just stretching, but I have treated and seen many badly damaged and dead guinea pigs, broken bones and internal injuries being common. Animals should be kept in pairs for their well being and castration of the male with a suitable rest before a gradual addition of a female would be the way to go. It's a simple enough proceedure for any vet and will lead to a happy existance for your pets. Good luck