Hi. I'm just seeing this thread today... congratulations and kudos to you for adopting Foxy...
Coat problems can sometimes result from medical issues. As she will be spayed and have a dental in the near future, I would highly recommend getting bloodwork on her, including a BAT (Bile Acid Test) to check her liver function. These tests can be run up to around a month before any surgery is done, but check with your vet.
If all tests come back good, then I would procede with finding the best food for her and supplements that may help her, with the vet's recommendations. Proteins are utilized by the body for growing coat, but any liver issues need to be ruled out as a higher protein food could make her sick.
Also, I have found out that my pups take better to supplements if I add a smaller than recommended amount to begin with, and gradually work up to the correct dose over a week or two. This allows their tummies and their taste buds to adjust to the new flavors and consistancy of the supplements.
When helping dogs who haven't been properly fed and cared for, even though we are in a rush to see them vibrant and healthy, the process should be done slowly or gradually. As my vet says, 'They didn't get that way overnight, and you will not be able to fix it overnight.' Until she has bloodwork done, it might be best to stay with a food that has a mid-range protein content of 18-22%. Adult dogs don't really require super high protein diets, unless they are working dogs (showing, agility, etc), or are extremely active.
Also with a few teeth needing pulled, she may have a problem chewing kibble. Soaking it in water to soften it may help her to eat better, if a tooth or two is bothering her. I belive kibble is usually more nutrient dense than canned foods, so soaking it would be worth doing, as opposed to feeding her canned food alone.
And because she is small, IMHO she may do best on a minimal vaccine protocol, such as Dr Dodds Protocol. I wouldn't allow her to have more than one vaccine at a time, and never if she has any fever, nor at the same time she has any surgeries. Safer that way to wait 3-4 weeks inbetween vaccines and procedures, and the pup will benefit more from each vaccine when given a month apart.
Here is a link to Dr Dodds Protocol:
DR. JEAN DODDS' CANINE VACCINATION PROTOCOL ? 2010