Although Jaybay has posted good information (as usual), I would hope that it would not be as complicated as all that. Since the second dog will be a puppy at the time of introduction your first dog will automatically be the dominant dog, at least at first, even if she did not have an alpha personality. I really don't think there will be any problem. The two will rely on each other and should get along.
When picking up the puppy you should bring your alpha dog to help with the choosing. As Jaybay has said, it will depend on the individual, not the gender. Although a male might be a better fit especially if you don't have the puppy desexed (spayed or neutered) at the right time. Though I really advocate spaying and neutering.
I'm not the vet, but in your shoes I would hire a behaviorist to help you choose your new dog. A behaviorist (not an obedience trainer) can identify traits in your current dog and find one with a compatible personality. There is a bit of an art to introducing new pack mates to one another - which a behaviorist can help you with so you get started on the right track. It's all about the individual personalities involved rather than breed.
I had a very dominant dog similar to yours, and she never did get along well with dogs outside her own pack, with one exception. She was a total nut for puppies! Once she made it clear to them that she was the Top Dog (took all of 2 minutes usually), it was off to the races and play time. You just never know sometimes. :-)