Hi. I adopted a teeny tripawd dog a few months ago. He's only 9 lbs. so this may not apply to your dog. I bought the convert harness for Stewie. It was just too stiff for him and didn't fit him well. The problems with it were definitely because of his small size. I returned it and after trying another harness that also didn't work well I decided to buy the Ruff Wear Webmaster harness.

The Ruff Wear harness is fantastic. It's almost like a vest. It has two good sized straps that go around his torso and one that goes around his neck, so it stays in place really well. It's sturdy and gives him good support. He's missing one of his front legs and when he lifts his leg to pee he sometimes starts to fall over. I can help him keep his balance by pulling up slightly on the lease as he's lifting his leg. I'm able to do that with this harness because it's so sturdy and covers more of his body than the other harnesses I tried. 

It took me some trial and error to realize that there are different fit challenges for small dogs.

Good luck!