I have been breeding rats and mice for the better part of the last 10 years. I have had mice foster rat pups, and rats foster mouse pups. Mouse pups raised by rats do get bigger (by around 7-10g), and have a much nicer temperment. On the other hand, rats raised by mice tend to be more mouse like, and the mouse may kill off most of her litter to feed the rat, and because the rat will get bigger than the mouse babies, so the mouse mom thinks her babies are sick and kills them.

Due to this, when a mouse has something I like in its color/pattern i will remove it from the mouse mom and place it in with a rat.
This makes the future mouse breeders nicer, and more used to being handled.

This talks about fostering...
http://www.afrma.org/rminfo8.htm
If you look for rat and mouse breeders that breed for pets not feeders you can find a lot more of this.