I'm bringing a 9 month old dog into our home, do I consider her an adult or a puppy when following your guidelines for establishing pack leadership?

Hi, I am very impressed with your website and have referred to it for advice many a time, but I cannot find the answer to this question:

I have a six year old, well trained, docile dachshund female. She comes to work with me every day as a therapy dog, and has met thousands of people. She also has several "dog friends" that she interacts with calmly and non -aggressively. if a large dog gets too "physical" in play, she runs to me, and asks to be picked up. She is crate trained.

We are thinking about purchasing a 9 month old female collie, that has been socialized and has even been shown a bit. Her owner tells me she is friendly and non aggressive (I havent met her yet). She is also crate trained and in fact is used to being fed in her crate and then napping in the crate for 2 hours after feeding.

My question is:

Do I consider her an adult or a puppy when following your guidelines for establishing pack leadership? i.e. Do I "isolate her" or treat her like a new puppy? I intend to follow all of your rules for introducing her to my doxie, but I don't know how to approach her for the first few weeks.

Thanks,
Deborah

Ps. Do you think it's a good idea to bring a 9 month old into a new home or is it better to start with a little puppy given my situation?