It's not a matter of what someone else let you eat, it's a matter of what food doesn't upset your digestion in an obviously negative way, such as fructose and quinoa or amaranth will do to me, as you are right that the starches in them break down to glucose, and it is even worse than pure glucose since it is harder to break down and the glucose could end up partially unabsorbed and become feed for nasty bugs.
Fermented foods is very gread in my opinion, and even important. Most vegetables contain many natural toxins, which are not important if eaten in small quantities, but they can become important if eaten in large amounts. Unlike cooking, which also destroy many of these toxins, fermentation not only preserves minerals and vitamins but it makes the minerals more assimilable and increase the count of most vitamins (made by the bacterias).
I would still avoid stuff like soy sauce, miso and tempeh, they're fermented with a non-toxic mold and are not really probiotics. The mold could cause allergic problems.
Sauerkraut is great and is relatively easy to make. You can ferment most veggies in a similar fashion, although some of the most common seems to be cabbages (green, red, or the chinese ones), daikon radish and beets, but it seems all can be fermentend, although some are more ideal for the growth of good bacterias. Cabbages naturally contain many good lactobacillus bacterias. Best done with organic veggies.
Fruits can be fermented too, but I'm not familiar with it at all, I believe it is too high in
Sugar and the results might be slightly alcoholic.
Yeasts can be problematic too, so kefir is to try with caution. There can be allergic problems.
When tolerated, kefir is really good. I would avoid making it with the common pasteurized homogenized milk; I have the luck to have access to an organic low heat pasteurized unhomogenized milk, ideally one should get organic raw milk, goat being one of the best.
In fact I will receive some kefir grains today!
I had experiment in the past with them and the results were very bad, but I was using at the time common 2% milk.
Commercial yogurt is not very good, as the bacterias count is highly decreased from storage, it is usually high in
Sugar (even when plain)