Ok, I see. Good for you!

Why are they called Designer mixes?

You know, I don't think anybody can really put their finger on how these mutts came to be called Designer mixes. The main point was that after the failed experiment of non-shedding guide dogs, the Labrador Retriever/Poodle mixes, unscrupulous breeders then saw their chance for $$$ and started breeding mixes, and gave them ridiculous names.

Why do Shih Tzus have a weird smell?

I don't have one, but I do have the breed that supposedly help build the Shih Tzu, a Pekingese. I think that the reason behind their odd smell stems from their thick double coat and skin oils, like your vet mentioned. However, a lot of how your dog smells comes from its diet, too. A dog on a high quality diet usually smells a lot nicer than a dog on Kibbles n' Bits.