OK, as a dog trainer, I've got to chime in here. If you train your dog to walk with you properly, instead of pulling, an ordinary buckle collar should not be a problem for the oridinary toy dog. BUT it is extremly difficult to train a dog to heel if you are using a harness! If you are dead set against a collar, I'd use a Halti or Gental Leader instead. Harnesses can teach bad habits!
Here's what I wrote in regard to another post on the same topic:
Unless I want a pulling action, I do not walk a dog in a harness. If I'm concerned about a trachea issue, which is a just concern with small dogs, I'd rather opt for a halti or gentle leader than a harness. These work by directing the dog’s head rather than pulling on the neck. I use harnesses for tracking or jogging or when I want the dog to pull against me. With a harness, you have very little control, its like driving without power steering. Many dogs learn some awful lead habits when they are put into a harness at an early age. A lot of people compound this problem by using those Flexi leashes to which they can not grab on in a pinch without getting a nasty burn. A simple six inch lead (leather or cotton are my first choice since they are comfortable to hold and do not cause rope burn) and a halti or gentle leader for a dog who had trachea issues.
http://www.yorkietalk.com/forums/showthread.php?t=1069
Just a thought . . .toy dog people get away with things larger dog people can't. You can walk a Yorkie whose pulling her little heart out at the other end of the leash .. . try that with a German Shepherd. But having control of the leash and your dog attached to it could save her life - think of all the possiblities - walking along a curb and car swerves out of control, a large dog or cat bounds in front of your path, a kid skids out on his bike . . . a dog who is attentive and knows how to heel is a pleasure and a lifesaver. It's worth the time to teach your dog to walk without pulling. Without the pulling = you can reduce the wear and tear on your dog's throat in the process too!