Thanks so much for the additional information April. I really appreciate it.
The important thing...maybe the MOST important thing to remember is that even though I'm sure you already love her...you are a stranger and she is in a strange place, so taking it easy with her for a few weeks, until she adjusts to you and her forever home. I'm sure you want her to feel comfortable and to know that she's safe...but these little rescue dogs can get overwhelmed by simple day-to-day things that you do with your other dogs pretty fast. Imagine if you were dropped in the middle of Russia...you don't know anyone, you don't know the customs, and you don't speak the language...you'd be a little overwhelmed, no matter how nice everyone was. This is what your new dog is dealing with.
I actually have a bit of experience in this situation. I am currently owned by a Maltipoo named Vinnie who spent the first four years of his life in a very small kennel in the backyard of someone who was breeding him to make money. He wasn't house trained, he didn't know how to be a pet and was horribly, terribly skittish around humans. My first night home with him, he bit me. For the first 6 months I had him, he was anxious and made 'poop art' (he defecated when he was left alone) in his art every day.
With my little guy, the first thing I did was get him a DAP collar to help with his anxiety over the new life he had. These are collars that are impregnated with a man-made version of the dog appeasing pheromone, which is a pheromone that nursing bitches give off to their pups to help them feel calm and secure. It's something that humans can't smell, but it has an amazing effect on dogs with anxiety and other issues. Although you can find them at your vet's office..you can also find them online at places like Amazon.com and Ebay for much cheaper. Just make sure they're DAP brand, as they seem to work better than some other versions.
I would also get her a regular, hard sided kennel for in the house. I'm sure you're a little concerned about her feeling like a crate could be like punishment..but to her, a kennel is safe...In the wild, dogs live in dens, so put the kennel somewhere without high traffic, cover it with a blanket and put some soft pillow or blanket inside, it is where she's going to want to retreat there to feel safe. My little dog was HORRIBLE until I got him a kennel...I put it in an out-of-the-way spot and when he was feeling nervous or scared, he would go there, knowing that he wouldn't be bothered there.
I'd also get her involved in a basic obedience class. Not because you want her to learn to sit and stay (although that's super nice too), but because these kinds of classes will build the bond between you and your new dog and will help her build self confidence. Almost all breeds of dogs love to have a job, and get a great feeling of satisfaction doing one...what you're going to learn in these classes you can take home with you. Telling her to sit might be a fun thing for you, but for her, it's a job. That will help her come out of her shell a bit.
Finally...make sure you keep everything on her terms for now. If she's timid, allow her to be. Ask new people to avoid looking at her, talking to her, or trying to pet her. For a frightened dog, having someone invade her personal space will be a super intimidating thing. Ask new people to just sit and ignore her. If she comes near them, they can toss her a bit of a tasty treat (My dog loves the freeze dried beef liver you can pick up at any pet store), but not to talk to her or try to pet her. Eventually, she'll associate new people with something good (the treats!!) and will be more likely to want to socialize with them.
I hope this helps...and if you have any other questions about your girl, please don't hesitate to ask.