Again, I thank all of you for your insightful comments...
gemy, YES !, I have personally witnessed that seemingly instantaneous acceptance into a Pack. They somehow seem to know "AHH, home, I'm happy now", and, "Companions and play, let's have fun".
So, the general consensus seems to be...
A medium sized Dog, like the...
Border Collie Page or
Australian Shepherd Page
Then, a Female, which might provide a closer relationship with smaller Dogs. ie: "Her Puppies" attitude.
Then, the issue of age...
Somehow, I just can't get past the idea that it should be a Puppy when getting it initially. That's probably because that's always been the way I've introduced a newbie into my Pack previously.
I have always respected seniority issues within the Pack, and encouraged my Males toward an Alpha role, though the Yorkshire is predominantly matriarchal. My Female Becca is the consummate Alpha Female, and though her regular role is the maintenance of relationships, will get right in my Male's face, if he gets too rough with the puppy. However, when a defense issue arises, she always calls in the "big guns", and assumes a supportive role. I can see this as a positive indicator, concerning her ability to allow another Female to step up to the defense of the Pack.
As you can see, I am very conscious of the "mental health" of the Pack. Maybe overly so.
Herding References:
AKC Breeds by Group - Herding Group