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?
Don't worry to much about this. The puppy may have already learned to do steps at the breeders house. When I used to breed Bichons and I was housebreaking the pups, I had 3 little steps off my porch & sooner or later the pups learned on their own or tried on their own!
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.
Give the cats time. Don't force the pup on them. They will want to check him out and may do so than hiss at the pup. Don't scold the cats, they may at 1st be scared & maybe even mad at you for bringing something like that into their house 
Give them time. My Siamese are famous for acting like dogs are a problem, but the first time you find them all cuddled together will be the best (be sure to take pictures!) My female cat will still on occassion try to attack one of the yorkies - usually out of jealousy. The dogs know just beware.
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.
You'll be able to answer this question better yourself after you have him in your home. They all have different temperments. I would just make sure that if the pup needs some down time/away from everyone, he gets it.
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
My female yorkie on occasion goes and picks up one of those furry cat toys you can fill with catnip - she thinks anything furry belongs to her! lol
Not knowing what you have for toys, its hard to advise, but I would definitely be careful if you have those little plastic balls for the cats that have the little bell inside. I would be afraid of the pup chewing on the plastic and swallowing or choking on the little bell. Once the dog is older you shouldn't have to be to concerned.
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...
Spoiling?? Nothing wrong there, but a puppy may not sleep thru the night with you right away. And housebreaking is so important 
- might want to conquer that 1st. Also, if the cats are sleeping with you, you might at least intially want to keep this time for them, so they know there is one point in the day they can have you all to themselves - because they will likely be jealous of how much time & attention the pup takes at 1st
6. Is there a certain brand of harness that anyone can recommend? I know that I'll need a harness for walking.
I'll opt out on this - I just bought something at the local pet store that came in the tinist size & I have it almost as small as it can go.
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.
I feed Flint River Ranch - I am so impressed with this product. My cats & dogs are both on this
8. Some friends have golden retrievers. Would it be dangerous to bring the yorkie to visit?
Has the golden been around small dogs? He/she will probably be really excited about a new friend. I would probably do a lot of holding the yorkie at 1st and letting the golden smell the pup & see how things go from there. I might be fearful of an excited big dog stepping on a pup. Also, in your lap the yorkie will be more on eye level with a golden - it would be overwhelming I would think for the pup at 1st
9. Are there any questions I didn't ask, that I should be asking?
How do I cure a YT addiction??
Sorry, I can't answer that one - I'm too sick with the same disease