Which Greyhound comb should I use for my Yorkie and Shih Tzu?

I would go with the Medium/Coarse. I use a fine toothed comb on my shorthaired cats. It would be difficult to get one through a coat with any length to it.

I don't think any of these combs is appropriate to be honest. Can you contact either breeder and ask what grooming equipment would be best?

They are both breeds with extremely fine hair. I would assume the third one would be most appropriate. However, if you don't brush them frequently they will develop mats pretty quickly. For that reason we always kept them groomed with short hair.

On long coated dogs I just use an anti tangle hairbrush. You can get them at the pound shops now.