Rabbits are social species so keeping at least two together is the standard. Mixed sex groups work well if they are neutered. Problems may arise when keeping a number of entire males together and neutering the males is advised in these situations. Spaying (neutering) is advised even in female only groups to eliminate the risk of them developing uterine cancer in middle age.
Keeping rabbits and guinea pigs together is no longer advocated. Rabbits can easily injure a fragile guinea pig with their powerful kick and bite. Rabbits are also frequent carriers of the bacteria Bordetella bronchiseptica in their airways, whilst it doesn’t appear to cause the rabbit any symptoms, guinea pigs can suffer from a fatal pneumonia.