I can't remember where I found the reference to culturing buttermilk, but since I make crème fraiche the same way (and you can do sour cream similarly) using cream, I thought "why not"? And it worked! (Yogurt can be done this way too if you put it in a small thermos and then into a larger one full of hot water - just let it cool 12-24 hours.)

And yes, the rice/almond flour combo should work quite well for most gluten-sensitives.