My Laya is about 8 lbs and I still can't use a collar on her. She immediately starts coughing and gagging. She pulls on her harness too. I was at a dog show and bought a wacky walker, and it actually worked for us. It is a rubber leash that looks like a bungee cored sort of. As they pull it pulls back giving them a little correction. I was skeptical but decided to try it and it made a huge difference in our walks.
wackywalkr.com