I have a 12 week old yorkipoo (see my avatar!) and 2 cats.
Elphaba ( the puppy) is 2lbs 5 oz right now. The cats are 11 and 17 lbs as of last vet visit.
Elphaba wanted to take off after the cats from the minute I put her on the floor in the house. She's been home about 3 weeks now...and she really wants the cats to play and they are not having it. The 11 lb cat will let the puppy get close if she's calm and they'll sniff each other. The 17 lb cat is pretty mean...though I've noticed he's the same with with his sister and they've been together since birth. Elphaba runs up to the cats stops short barks and runs away trying to get them to chase her. The one cat just lays there and the other kinda growls (yes...didn't know cats growled before this) at her. The big cat has also taken a few swipes at the puppy since I got her....usually there's a little yelp but no injuries. I am thinking it might be time to declaw the kitties though just in case....Elphaba will only be 4-5 lbs max...and the cats could really hurt her if they wanted to. for now I'm just keeping a REALLY close eye on them whenever the puppy is out of her crate.
Which leads to training. I've never had a small dog before, always 40 lbs or more, so she's my first little baby. Crate training I think is a necessity for ALL dogs. She wasn't crazy about it the first day or two, but now she doesn't make a sound when I put her in. I'm working on teaching her its her bed by saying "bed" when I put her in it. If she's like an old dog of mine, eventually I'll tell her to go in her bed and she'll go.
If you do crate training the way its meant to be done its the best way to housebreak a dog. Problem is many people don't do it right. They feel bad giving the puppy no extra room in the crate and having the puppy in the crate most of the time. But this will teach the puppy to hold it and since she goes from the crate straight outside EVERY time it will teach her to go outside. Also, a schedule is good. After eating we wait 10 minutes and then go for a WALK so she can do her business. And when she does her business I always say "good potty" lots so she learns what potty is.
Personally, I've never used puppy pads and am against them. I know lots of people start with the pad near the door and then move it outside the door eventually, but everyone I know personally who used pads continues to have trouble getting the dog to go outside. Some people will say, but what if you aren't home and the puppy has to go, shouldn't they know WHERE is okay? My answer is no. No where inside is okay, and the puppy pad teaches it is. The only think OK is for the puppy to learn to hold it until they can go outside.
Also, watch for the signs. Elphaba has a distinct poopoo dance. So as long as I'm watching her when she's out there is no poopoo in the house. I'm having a harder time recognizing when she needs to pee...so whenver she's sniffing a lot outside we go.
She's been home three weeks and so far its going well. She sleeps through the night with no mess in the crate. Occasionally she wets the crate during the day, but its infrequent. And its been 2 days since we've had an accident in the house. I think thats pretty good for 12 weeks old and only 3 weeks with me. But I know we're far from done!