My little girl has tiny bird legs and is hairless on her leg and entire underbelly. I find what keeps her the warmest is layering.. A t shirt, then a sweater, then a jacket. I am actually surprised she can walk!
But I am a horse trainer so we are at the barn all day and into the evening in all weather. No one wants to stay home so I have had to find ways to keep everyone warm and dry. I even tried these little doggie leg warmers but she was having none of that!!!
I think wet, cold rain is worse than drier snow. Except for the paws, of course! But most important is not letting them get too cold for a long period of time or go too long without food (mine always eat 2 to 3 times if we are out on a cold day).
Good luck! It sounds like you will do right by your little one! Sometimes they surprise you by what they can do! My 6+ lb chi can out hike me!!! He never seems to tire!!!