I would never ever bring a second dog in unless the first dog's behavior was really good. I agree with other posters that you should work on basic obediance, including NILF and I also wouldn't leave a new dog alone (puppy or rescue) in a situation where they could do damage. We crate trained our dog until he was about 2-3. If we weren't in the room with him, he was crated.
I think you should handle this dog just like if it was a little puppy, because if she wasn't trained, she has no way of knowing proper behavior.
::spank spank to the old 'owners' who neglected her, and hugs to you for saving her!::