Can you substitute carton coconut milk (e.g. Silk) for canned light coconut milk? Making soup.

Coconut-based beverages (like the Silk stuff in cartons) are much, much less rich than even light coconut milk, and also have a lot less coconut flavor. If your soup is intended to have coconut flavor and/or richness, such as in tom kha soup or something like that, coconut-based "milks" won't work well as a substitute.

I would just make sure to read the ingredient list very well. Sometimes the carton milks have extra sweeteners and stabilizers that might throw things off.

Here's a Food52 article that may be helpful: http://m.theweek.com/articles/442714/cook-coconut-milk

Yes.