Lab Rescue from a Puppy Mill

Question
I just rescued (through a rescue org) a 2 year old female Labrador who was a breeding dog at a puppy mill.  She joins our family which consists of my fiance and our 3 year old male mini dachshund.  I have a few questions if you don't mind.
1:  I currently gate her in our kitchen when we leave the house.  Our little guy can move about the whole downstairs.  I am concerned she is having separation anxiety.  She is not destructive.  In fact she is terribly submissive.  However, she spends her time pacing, panting, and standing up to look out the window.  I can't know for sure, but I don't think she rests.  We just got her a few days ago.  How do I get her comfortable with our leaving her at home alone?
2:  My little guy is somewhat dominant.  He has taken to her very nicely, which has been a very pleasant suprise.  However, he feels he needs to always go first (for instance, walking down the hallway - he must be first)  If he isn't he will bark at her.  So, he does bark at her when quite a bit.  I intervene.  Should I just let them work out their boundaries, or should I correct my mini dachshund?  I'm just concerned he could become nasty with her.
3:  I am concerned about their size difference and whether or not ever leaving them home together without a gate separating them will be okay.  She jumps up on all the windows and comes down without looking for him.  I'm afraid she will hurt him by accident.
Thank you so much!!!!

Answer
It does sound like your dog suffers a bit of anxiety,sometimes it helps if you take her for a long walk before you go out(if you have time)so then she becomes to tired to pace,sometimes they get over it,and other times if it is really bad,you may have to speak to your vet about medicating her

As for your other dog being dominent,it's really best to let the two dogs sort it out themselves unless they are hurting each other,it's much better for them to sort it out

And now for the size difference,lots of dogs of different sizes live together,and it's a good idea to seperate them when you are out though,just in case,because of the sizes between them,other then that,it's not a big concern