Gerbils and mice are both small rodents, but that doesn't mean they can live together. While many gerbil species exist, only Mongolian gerbils (Meriones unguiculatus) are commonly kept as pets, although other species are occasionally available. Domesticated mice are known as fancy mice, but were originally bred from the wild house mouse (Mus musculus).
Gerbils and mice shouldn't live with each other in the same enclosure. As they're different species, they're likely to become aggressive and territorial toward one another, rather than forming a social group. Gerbils are larger and stronger than mice and can injure or kill them when they fight. Mice and gerbils can both carry diseases to which they're immune themselves, but could pass on to each other. For this reason, they shouldn't even be housed in the same room together, let alone the same cage.
Keeping Mice in Groups
Mice are extremely social rodents that form complex communities. As such, they should always be kept with at least one other mouse, but preferably in even larger groups. Unless you want to become overrun with babies, ensure that you keep them in single sex groups. Most mice, especially males, are more likely to get along with each other harmoniously if they've been together from a young age. It's best to buy a group of siblings who have lived with one another since birth. Adding or removing group members at a later date can cause problems.
Keeping Gerbils in Groups
Since they live in large colonies in the wild, gerbils need to be kept with one or more companions. A lone gerbil will only become unhappy. Adult males will sometimes fight, so inexperienced gerbil owners are best off getting females. However, siblings who have always lived together are more likely to get along. Although you rarely find any species other than Mongolian gerbils for sale, it should be noted that gerbils of different species shouldn't be housed together.
Gerbils or Mice?
Since you can't keep mice and gerbils together, you may need to decide which of the two is the right pet for you. Mice are smaller -- generally measuring no more than 3 1/2 inches compared to gerbils' 4 inches. Three to four mice can generally live happily in a 10-gallon aquarium, whereas only two gerbils can live in an enclosure of the same size. Gerbils have a longer lifespan, often living for up to four years, whereas, mice generally live to between one and three years. Gerbils are quite gregarious and can be easier to tame than mice, who are often timid and wary of humans.