Thank you all very much for the helpful advice. I'm not going to respond to each post individually because that would be a very long post!
I read all of your posts and have been thinking about this all day (as has my wife). We also asked our vet tonight, and he thought that it would most likely be fine, but said that we should make sure to have trial periods and a slow introduction. Right now, our conclusion is to go forward, but we will be very careful about how our current girls interact with her. If the adoption place will allow it, we'll definitely have at least two meet-and-greets to see how the dogs get along, and then we will try to foster the dog for just a weekend, and then for a longer period of a couple of weeks, before we commit.
As far as them turning on each other because they are female, that is definitely a concern. The vet said that the risk was probably small as long as you are careful. I'm willing to tolerate a little bit of risk, because we really like the dog that we found (assuming that we can even get her).
To address the concerns laid out in this thread, we are thinking:
- Slow introduction - meetings with current dogs, weekend trial period, foster period before adoption
- Always keep the dogs separated when we are not supervising (forever, not just in the trial period). Our current thought is to crate the new dog in the kitchen where the other girls stay, although we'll have to see how she reacts to that. That way at least they would be unable to harm one another.
- Up our walking time to at least 45 min/day or, if that doesn't seem to work, get a dog walking service or do doggie day care (preferably the walking service, although there is a doggie day care close by).
- Strictly follow the procedure outlined in the dog training book that we have (Sylvia-Stasiewicz, "The Love That Dog Training Program") (the book was a gift, but so far it seems to work -- I'm definitely open to other suggestions, if anyone has favorites)
We are not totally set on this, it is just our current plan.
Thanks again for all of the help. Any more thoughts, comments, and/or criticism is welcome!