I wouldn't use a harness that goes around the neck like a collar. But even with the ones that don't go around the collar, your yorkie might get their lower jaw hooked around it and start chewing. Tia's harnesses used to always be tightened correctly, but she figured out how to roll out of it when we were on a walk, or chew on it when left to her own devices (I went on holidays and she was being sat by a friend who left the harness on her for a while after the walk...Tia chewed through it).
I recommend ordering a harness from chattiesmom. Tia chewed through hers but I believe it was due to anxiety that I was on holiday (at elast that's what I like to believe
). So I replaced it with a Buddy Belt. Its made of good quality leather and there is NO WAY for them to chew on it while it's on. They are a tad pricier....but I think it's a good investment. Also, the girl at the store fitted her to a size 2, which to me seemed tight, but sure enough, the leather softened up just enough to form to her body and her BB has never bothered her one bit. Plus I know she can never "roll" out of it.