I'm sure there have been cases where rats housed with mice have killed them- but I've known some rats (mostly large, aggressive adult males used to living alone or with a set group of 1 or 2 other rats) that would kill other rats that they didn't know. All I can tell you is in my personal experience, a normal, non-aggressive rat will not usually attack a mouse. Where I work we have a tank on the front counter where we will put a dozen or so feeder mice for quick grabbing so we don't have to go all the way to the back room to get them. Occasionally a rat will get tossed in there too if we grab the wrong size accidentally or some such and are too busy with customers to take it all the way back to the racks immediately, and I've never seen an incident where these rats (usually juveniles, used to living in a large group of other rats) have attacked the mice. Yesterday I even saw the rat we'd put in grooming one of the mice! However, this is just my and YMMV, I'm sure others will chip in.