Grammatically speaking, it should be "mouses" since it's not actually the same as the animal. When you use the word "mice" you're not so much talking about plural but plural
grouping which is an animal thing.
i.e. you could have one goose, but a gaggle of geese.
The really fun one is fish. One fish is a "fish". Many fish of the same kind are "fish". Many fish of
different kinds are "fishes". heh
Ain't English grand?