My [biological] sister, our friend and I used to be roommates before I moved. I had a cat named "Xanthia" and my sister has a salamander named, "Perris". Yes, a salamander. It's like a gecko but doesn't climb walls - we found it in our swimming pool some 10+ years ago. Anyway, she keeps it in a terrarium like environment. It's a fish tank with pebbles in the bottom and a tub of water for him to go swimming.
There was no lid on the tank; but the tank was up on a shelf some 5ft off the floor. Xanthia used to jump up there and want to look at him. My sister was afraid that the cat would eat him but she never did. One day I heard my sister panicking and I ran in to find that Xanthia had jumped *inside* the tank and was sitting in there with Perris. Not eating. Just looking.
Later, I got another cat, "Bella" and Bella does the same thing. She just likes to jump up and have a look.
So, I suppose it really depends on the cat. If I was babysitting some of my friends' cats I would definitely take extra care because they strike me as the kind of kitty who would want to make Perris lunch. However, neither of my cats were interested in eating him.
If you adopt the little mouse, then I would suggest that you make sure that the cats are put away in a bathroom or something if you need to clean the cage or want to play with him. Even if you let him out to crawl on your shoulder or something, I wouldn't put it past a cat to creep up and snatch it right out of your hands. If your cats don't seem to care, then you probably won't have too much to worry about!
.....Kelly