Don't even think about taking prebiotics in the beginning of treatment (Kill phase). If you are diligent in the kill phase with diet, kill supplements and enemas you can proceed to the restore phase. At that point the fungal load is reduced sufficiently that adding probiotics is appropriate. I would still take kill supplements and do enemas to keep candida at bay. But after taking probiotics and fermented veggies for a week it would be good to gradually add a prebiotic. In my case I went with lactulose which can be bought on eBay if you live in U.S.A. or OTC in Great Britain. I started with 1 tsp. every 12 hours and then after 3 weeks went to 2 tsps. every 12 hours. This will ramp up LABs (lactic acid bacteria) such as lactobacilli and bifidobacteria which produce lactic acid in the colon making is slightly acidic which then causes the candida to become bud or spore form (benign) instead of hyphael form (pathogenic). It wasn't until here that I started to feel like I was progressing against candida and not at a stalemate. Don't expect an instant cure at this stage and don't jump the gun and introduce sweets or grains to the diet. That would be a mistake.
Dvjorge has found several clinical studies showing how lactulose causes reduction in candida to almost undetectable levels over a fairly short time. I experienced it myself. I can't find studies on FOS or GOS related to this but I speculate that if candida has been reduced sufficiently and probiotics are introduced in high quantities, taking FOS or GOS will feed probiotics so that the change in colonic pH will reduce candida. I know people freak out about FOS saying it exacerbates candida but I think that is true when candida is out of control and very little good LABs are present in the colon (cecum especially). I think that if you reach a critical tipping point where LABs outnumber candida by quite a bit, taking prebiotics only pushes that ratio even further toward LABs. So the whole point of the kill phase it to reduce candida way down.
So if you want to be absolutely safe, go with lactulose. I've also heard people being cured of candida by taking potato starch which is a resistant starch 2 Tblspns twice a day. Again, I haven't tried it so don't debate me about it.