Firstly, when you adopt a kitty from a shelter, it's a good idea to confine the kitty for at least a week and make sure that he/she sees a vet for testing before being introduced to the current brood. Going 24 hours without food is sometimes OK, but not drinking any water will lead to dehydration. How is her output? I would definitely get her into the vets again. If you don't trust your vet, then I would suggest a second opinion.

You sound like a very caring Meowmie and I wish you good luck. Please keep us posted.