Just wanted to offer my experiences, when I was brand spanking new rat owner and had rats and mice together, they got along splendidly. Never had a single issue, they were bonded. Slept, ate and explored together happily. The 3 rats that grew up with 2 mice loved them to death, however any other rats that did not grow up with the mice made it clear they would harm them in a second. I think it is very much the growing up with them that matters, otherwise it would be nigh impossible to not have a bad reaction.
Reading through some of the earlier replies it almost made me feel kinda sad so many people were adamant about separating them without having the necessary information on bonding and behavior with eachother. Sometimes animals know much better than their human caretakers and I'm so glad it worked out excellently for the pair in the end, as it did for my 2 girls with my rattie guys. So just wanted to offer my experiences, it serves to note predator/prey relationships can frequently be bypassed by being raised together. While it isn't always wise at all, mice and rats are similar enough that no issues should arise.
In the end, the only negative from that crew of mixed 5 was the rats suffering a period of depression when the mice passed away and were absent from the cage. But that in itself is normal for any intelligent creature.
Note: I haven't been on the forums in years as I finally stopped adopting ratties after having 15 total over several years but when I got the alert for this thread in the email today I had to stop in a comment.