I had my ferret and rats in the same room for a while with no problems. If you are super, super careful, it can be done. We eventually moved the rats out to the livingroom, not because of the ferret, but because it was easier to clean their cage and around their cage out there. When Riley was out playing, he never bothered the rat cage, and the rats only came out to play in the bathroom, shut off from the rest of the house, so I never had problems. That was because my boyfriend and I were always very aware of what doors were open and shut, have a dog that gets along great with the smaller animals, and we always knew who was out at what time, though. If you live in a household with many more people and animals, it gets trickier.
Whenever I handle the rats and then go visit the ferret (or vice versa), each animal sniffs my hands eagerly, but shows no signs of fear or stress. I think they are just curious about the new smell, then they move on to other things.
