Despite their common names, African Soft-furs (ASF) are neither mice nor rats, although they do belong in the same taxonomic family.
They grow to be about twice the size of a mouse and, in fact, can happily live with fancy mice. Both species will eat, groom, and sleep together. Their care requirements are identical to fancy mice.
ASF are often described as being more "bitey" than your average pet mouse. Ours have been bred for temperament and are less inclined to bite than most pet hamsters. Our females even allow us to pet them when they're nursing. That being said, they do not usually enjoy being handled so we would not recommend ASF for novice rodent owners.