An 8 week old puppy, female, may come in and take over immediately. Been there, seen that! With dogs, adult or pups, "ya just never know" what the relationship will be till it happens.
The better socialized a dog is, the more positive interactions he has had with other dogs, the more likely he will welcome a newcomer, pup or adult. Be prepared for days or weeks of introductions, have a back-up plan in case after a couple of months it still isn't working. Be as relaxed as you can be, dogs really pick up on our emotions and fears. A puppy might be great, a young adult may be wonderful too, already housebroken, good walking manners, but still a playmate. And till you have 2, you will have no idea who the 'alpha' will be, but they work it out and area happy once resolved.
I have 3 now and love it, doubt I will ever have less than 2. They love me, but love each other so much it's wonderful to watch!