I've been having 2 teaspoons of FOS daily for a few months, with good results. Also, a lot of the low carb special products I use have inulin or FOS in them, including bars from natural lowcarb store, and cavalier stevia chocolate. Not sure I know the difference between FOS and Inulin, but it all seems to come from chicory root.
I have been borderline IBS in the past. The FOS makes me very gassy, but the benefits outweigh this - I have combined with high dose probiotics, Kombucha, kefir etc, and feel my bowel is healthier, and the change in my gut microbiome has been one reason, I think, why my BG's and Hba1c have come down.
I find that if I exceed 20-25grams of FOS in a day, gut gets a bit dicey.
I am aware that this is the complete opposite of the FODMAP diet, but my priority is my diabetes.
I use Lamberts or Biocare FOS powder, buy on the internet, can also be used in baking, home made low carb ice cream, pleasant sweet taste.