Whenever I get a new one I always weigh them before putting them in the tub to settle. I figure they're already stressed out, so a quick weigh in can't stress them any more than they already are.

You should move her to the weaned rats when she grows to the point where the rats are about 10-15% of her weight.

Until then, she'll be fine eating the large mice.

After she has eaten a few times for you, you can get her out and get a good weight on her, then go from there.