If you currently have a cat and are thinking about getting a Yorkshire puppy, you'll want to assess how your cat does with other dogs. It is best if you can see how she responds to small toy breeds and of course, it is best if the dogs that you are introducing her to are known for being good with cats.
2) If you are planning on getting a cat from a shelter, do ask about her history. The shelter may already have written notes concerning what type of home would be best and if she was already accustomed to dogs.
3) Plan for a slow introduction.
It's never a good idea to 'get things over with'. Even if both animals have a history of tolerance, they have not yet proven anything in regard to each other. If a cat is coming into a Yorkie's home, it is best to place the dog in another room (do not make it seem like any sort of punishment, have someone stay with him if possible to entertain and keep him busy) and allow the cat to explore the rest of the house, picking up on scents and getting a basic vibe.
If a Yorkie is coming into a cat's home, keep her out of the way while he learns about the house and gains his bearings.
After 20 minutes or so, each animal will be aware of the other, even if they do not see each other. With the Yorkie on harness and leash and the cat given a route to scamper off if she wished, introduce the two.
4) Signs and signals.
A clear tolerance, of course, is best. However, essentially ignoring each other is also not a bad sign; this simply means that neither sees the other as a threat.