I know dozens of people who breed mice and rats in the same room. Racks will be side by side with one another with no adverse effects to their knowledge.
However, I work in a research lab. The information given to me during orientation/learning period was that mice tend to stress more and breed at a lower rate in the presence of rats. So they have separate rooms for different breeding species (Also to prevent cross contamination, etc.).
But like I said, I haven't known breeders who have had any negative effects. Maybe try housing your rats and mice in the same room for a full litter cycle and see how your mice adapt.
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