CGurl,
Your comment on Yorkies ability to Guard is very interesting, though I have a different viewpoint on that as well (of course
)
Think of it this way...
Just how big and bad of a Dog, would you have to have where it couldn't be overcome by a Human ?
We can throw things at them, hit them with sticks, shoot them with Guns, throw covers over them...
there's just NO WAY that we can't come up with some method, where the Dog looses. Unfortunate, but, true !
So, why even try, don't put them in that position, when the Dog comes in contact with a Human.
If the Human is prepared to deal with an aggressive animal, the Dog is going to loose BIG TIME! so...
Why not teach/train the Dog NOT to do that..
to keep back, and use it's agility and tenacity, to make the biggest ruckus it can, and evade being captured. THEN, it's on and on and on, until the Bad Guy will just gives up and goes away !
That's what I do with mine...
If anyone (including me) tries to reach down and grab one, it dodges and retreats, and will not allow anyone to pick it up.
HA, sound crazy...I can just hear this...
"
WHAT, you mean you can't pick up your own Dog !!! 
"
I can, BUT, I have to give it a "recall" command first.
And, it's not anything that is normally used by the average Dog owner, or that another person would use, to get a Dog to come to them.
So, mine are free, staying well back, barking like hell, and will hang in there till the "threat" retreats.
This approach make Yorkie,
Chihuahua, and other small Dogs, a bit more attractive when it comes to "Guarding" ?. So, the a, b, c of this is... a) Don't put them in a position to make complicated decisions. b) Take advantage of what they're good at, Barking and Running. c) Use their tenacity to continue till the threat is gone (territorial).
Just IMHO
Have a good Day !
S.W.