Rats predate on mice; they can and will kill them. It may not be immediately, but eventually the mice will be attacked. If it's just for a very short time they may all be too busy exploring the new smells and sounds to bother with each other, but I wouldn't leave them for more than an hour or so personally.

Mice also carry an infection called Sendai that they can carry asymptomatically that will infect rats; it may not matter if you are feeding them off immediately but Sendai can decimate a rat colony very quickly. I keep my pet rats and feeder mice in totally different areas of the house and disinfect after dealing with the feeders to prevent any evil little mouse viruses from getting to the rats.