I own the Starter Edition on Steam and it had me enter the CD Key into the mogstation to get started. Now, I see that Shadowbringers is on sale from Square's online store for $20. If I buy it there, will I be able to use it in the mogstation since my Starter Edition is owned on Steam?
You cannot mix Steam and the freestanding PC version. Once one is registered to your account, your account will only accept expansions from the same.
Nope. Once you purchase a license, you are stuck with wherever you bought it. Since you bought the base game through steam, all expansions must be bought through steam as well.
I found this out the hard way last year. I bought the basic game through Steam and decided to purchase Shadowbringers and all past expansions. Square was having a discount on all games but I would have needed to purchase the basic game again and start all over. Steam did not honor the sale...