She sounds awfully young!!!! With no teeth, I would go to the pet store and pick up the puppy formula to feed her. She'll need to be fed every couple hours (yes even through the night!). My suggestion is to take her to your vet's tomorrow ASAP and have him/her check your pup over...Wow what in the world was the person selling the puppies thinking?? Even at 6 weeks she was 2 to 3 weeks from when she should have left the litter!!

Keep her warm with a hot water bottle wrapped in a towel and if you have a heat lamp throw that on too!! I'd watch the pup and the Westie together just as you would any puppy...

Good luck and keep us informed on how she is doing!