An excellent question. I am very suspecious of FOS, but I have recently taken a probiotic formula that contained a great deal of FOS. FOS can not be digested by humans, it is therefore digested by intestinal bacteria, and yes, as a necessary byproduct of bacterial digestion intestinal gas is generated.
I did not experience a candida flareup as a result of ingesting a great deal of FOS. Therefore, I am willing to give credence to claims made by FOS sellers that FOS does not feed candida. The FOS probably is feeding the benefical lactobacteria. However, since there are several hundred species of micro-flora living in the human intestine, the question is, what ELSE besides the lactobacteria is being feed by the FOS? So, taking FOS may be a bit of a gamble.
Maybe it's just me, but given the choice of a temporary case of flatulence and an on going case of candida, I'd opt for the former everytime.