I would not use a bark collar on these little guys!!! Them barking at your neighbors is a way of them protecting his yard. When you hear them barking, couldn't you bring them inside? You could also give your neighbors/kids/mailman treats to feed him through the fence...Once they associate your neighbors and mailman as a good thing it may tame the barking.
Yorkies are yappy dogs. It's just the breed.
He's just saying look at me!!! Pay attention to me! Or stay away! This is my yard, I'm protecting my humans
Good luck!