[SWHouston]

There are so many variables involved, it's hard to set any form of rule/procedure about this, but, here's what I do, and I know that in all instances, it's not possible. Anyway, when adding one to existing...

Same Breed.
Opposite Sex.
New Dog younger than the "resident".
then...
I try to wait at least two years, before another addition keeping a gender balance. ie: alternate gender.

I'm just one of those people, who respects/attempts to maintain the "Pecking Order", though many don't pay much attention to that. It's specially important with Females, allowing immediate and apparent knowledge to the new animal, that there is an Alpha present, and the new guy should wait their turn and observe it's place.

Yorkies are much like Lion Prides, in that the Female does a lot of the "management" within the Pack, and the Males just lay back, wait on the Females to catch something, or something come along where they have to protect the territory or Pack. That's probably why the Male mentioned before, doesn't get involved with the Female "battle".

But, getting back to the point, it's a lot easier if the resident is older, bigger, stronger ect... then there is a LOT less resistance of the new one to accept the others seniority, and...

When it comes to Yorkie Females, there WILL be an Alpha.