Hi, and welcome. It's nice that you were able to take this little dog in. It is good to brush her at least a couple of times a day. I usually use a small metal grooming comb to get the tangles out and then brush her with a pin brush. I'm putting a link to a page that shows various pin brushes. I have the small pink one. There are also various sizes of metal grooming combs on that site that you can look at.
toplinepet.com
People usually have baby teeth removed when they have their pets spayed or neutered. No sense in having them put under any more than necessary. I know dogs that are 4lbs have been spayed and neutered. It's best to make sure your vet has experience working with small dogs.
Would love to see some pictures of your new family member!