I have Don't Starve already from two vendors, Steam and Humble Bundle. Will I be able to buy Reign of Giants from here and use it for my Steam/Humble Bundle edition, or do I need to rebuy the game from here with RoG?
I haven't tried it, but my guess is it might work with the HumbleBundle Standalone version, but not with Steam. There shouldn't be a big difference between GOG's and Humble's standalone version.
The problem is, that you can't redeem and download the DLC on GOG, if you don't own the base game. If you're trying to buy it, you can only do it via gift code and only somebody owning the base game is able to redeem it.