I had a fleece coat for my purebred Sheltie, Scotty. He had no undercoat on his back or sides. Now with his new meds, he's starting to get a little undercoat on his back. Sides are still VERY thin. I'll play it by ear. If it's really cold, I'll probably put it on him.
Rascal, my Sheltie/Pom mix, has hair out the wazoo! But he acts like he gets cold and wants to sit in my lap when he comes in . Now bear in mind that it's been no colder than 50 since we got him.
The incoming Sheltie, Tigger, has more hair than Carter has liver pills so it's doubtful that she'll need one.
But if they do, Mama will pull it out of inventory and fix 'em up.