Do mice and rats fight each other?

Yes they do rats will eat mice in the wild.

But tame rats will actually raise the young of mice when mixed in with their own young as the mouse grows up the rat accepts the mouse as one of its own and will not eat it.

We did this while studying animal behavior. The rat was the only rodent not eat the young mice. We also tried the study with gerbils and hamsters and both found the young in the nest and ate them. These studies were preformed with orphaned animals in an attempt to save them from certain death. The ones with the rat as a parent grew and thrived right along with their rat siblings to a ripe old age.