I thought of this due to coconut fiber, though a search makes me think it probably doesn't apply if the fiber is insoluble.
But anyway, there's got to be particular bacteria that do a better job of processing certain foods. Tropical bacteria likely do a better job with tropical foods. Does this make any sense, or does it not really matter that much? I vaguely remember something about sea-weed and adaptations involving bacteria in Japan.
Certainly, there would be a lot of overlap, with much of the gut flora perfectly happy eating the same fibers, but are there particular ones that run optimally on more esoteric ones?
I imagine that with any consistent diet, you are creating an environment in your gut where certain bacteria are more likely to thrive. Given enough time and bacterial exposure, the bacteria best suited to your specific fiber intake will naturally occupy your gut.