I was just wondering if it is common for cross-breed dogs to suffer from SM, or is it usually just pure breeds?

I have a 3yr old female KC/Shih-tzu cross (adopted her 3 months ago, so still getting used to what is "normal" for her), and she's doing a lot of things that make me paranoid she has SM. She will sometimes stop during a walk to scratch at her harness, and will scratch at her ears and face at home, and push her face along the rug. She scoots along on her bum a few times a week too (have had her anal glands emptied within the last few weeks), and lip-smacks quite a bit. I've never seen her fly catching or air scratching though. Just wondering if I'm being completely paranoid, or is it possible she could have it? I've mentioned it to our vet a couple of times, and she doesn't seem to think it likely at this stage. As we've only had her a few months, I'm really not sure if all of this is normal for her, or something I need to worry about...