I can't imagine giving a puppy away after only 3 1/2 weeks! Poor little baby must be so confused. Good for you for taking her and keeping her from a life of breeding. Scarlett was 18 oz when we brought her home, and that was scary tiny to us, too. I just can't imagine someone letting a 7 oz/ 6 week baby go
Thankfully, you've got experience with a smaller pup so you know what to look for and what to protect her from. I'm sure in no time you'll have her bigger and stronger and running around with your other pups. If not, send her here and I'll fatten her up- We've got no problems with weight gain in this house