We went through a stage of having a rabbit and a guinea pig. They make good companions.
Erm, actually, no they don't and it is no longer advised to keep them together (unlike in the 60's and 70's).
Rabbits and guinea-pigs are very different mammals with different needs. They need a different diet (eg GPs need daily vitamin C), GPs like a warm environment while rabbits cannot tolerate too much heat, they communicate in different ways and GPs are often bullied by rabbits and retaliate by giving the rabbit a well-deserved nip which can lead to abscesses.
It is not a good idea to pair the two together but to pair like with like - GPs in small groups (females get on well together) and rabbit in compatible, neutered pairs ideally male-female or two females together as a second choice.
Pairing rabbits and GPs is rather like keeping a polar bear and elephant together simply because they are both large animals - they have very different needs!