You've gotten good answers already. Just to add my 2 cents...
1. I live in a townhouse, so I have stairs in my home. Should I keep my puppy in a playpen most of the time to avoid it getting hurt on the stairs? I'm guessing it may be a good idea to keep it in the pen in general for a while. Any certain age when I could stop?
A pen is a good way to keep them safe when not giving 100% of your attention. I would block off the stairs for a while. While puppies shy away from them until their ready, they're also goofballs who don't have total control of their bodies. One misstep near the steps will send one tumbling.
2. I have three cats. Would the puppy do ok with them for the most part? The cats like to tackle each other, but none are overly aggressive. None have ever been around a dog before.
You'll be able to tell us in a couple of months. 
3. I'll get my puppy around the first of November. Would it be safe to take it visiting when I do the family holiday stuff? I'll have had it a month by that point, but I don't want to expose it to anything bad. I would like the chance to socialize it though. I do know I'll have to be extra careful if children are present.
Make sure that he has all of his shots. And I agree that you should take your kennel with you. There is going to be a lot of stimulation around as well as careless feet! If nothing else, the puppy will need a nap.
4. Are there any types of toys that would be dangerous to a puppy? I have an absolute TON of cat toys that end up laying around, so I'd most likely need to do a puppy-check on all of them. I'll need to puppy-proof my entire home, actually. O_O
Puppies can make toys of everything. While you're checking your toys, be sure to puppy proof your house.... wires, shoes, jewelry, sharp corners.
5. Is it safe to let the puppy sleep in the bed with me? Should I avoid letting it until a certain age? I usually have two cats asleep there already...
Sure, but like mentioned before, be sure to housebreak him first and then work on training him to STAY on the bed through the night.
6. Is there a certain brand of harness that anyone can recommend? I know that I'll need a harness for walking.
I don't have a favorite brand, but kudos to you for knowing you need a harness!
7. Same for food - any recommended brands? Right now I'm leaning towards Innova, since I feed Innova EVO to my cats and they're all incredibly healthy and have wonderful coats.
There's a great recent thread elsewhere on food. It gives a lot of YT'ers opinions!
8. Some friends have golden retrievers. Would it be dangerous to bring the yorkie to visit?
Just make sure that the retrievers are well behaved and well socialized with small animals/dogs.
9. Are there any questions I didn't ask, that I should be asking?
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You're obviously going to be a GREAT Yorkie family!!