What is the best practice or easiest way to import product types from their content type derivatives?
It sounds like you have your "content type" confused with your "product type". With Drupal Commerce, you need to add "products" which are of a particular "Product Type". Each product has at least a title, a SKU, and a price, but you could have more fields like size for example. You then attach the products to a product display node so that they are visible in the shop. A Product Display Node is a regular content type node with a product reference field that points to the product type entity.

You may be asking why do I need both product entities and product display entities? This is for the situation where you have multiple products included with each display. So if you have a clothing article that comes in small, medium and large sizes and is also available in either blue or green, then you have 6 SKUs for this clothing article. You would have 1 product display entities with product references to the 6 SKUs.

Your content type that is the product display should include the name of the product, the general description of the product and maybe an image of the product, etc. Your product type will have the SKU, the price, and any other fields that are unique to the SKU.

Read more (and probably better constructed examples) at the Commerce Guys site