Ah it's a tricky one that. My brother had an adult moggy and took on an old rescue German Shepard that was supposed to be cat friendly. They seemed to get on Ok for a couple of weeks but one day the dog just flipped and killed poor Eddie
But I think that as long as the dog is a puppy (and Shelties are tiny little things) then you'll be fine. The cat will make sure the puppy knows who's the boss of the house. There will be a few weeks of tense standoffs and eventually they'll be fine - even friends.
Think about getting some Feliway spray to help calm the cat down.
Have a look at this:
YouTube - Sheltie and Maine Coon Wrestle
Ian F.