I've fished the Poudre as it exits the park well into October and it's excellent. The fly rods will be well worth the weight. The Cutthroat will be in their best fall colors so bring a camera.
I've done very well that time of year with #14 & 16 Lime Trudes. Pocket water is particularly productive in the fall. I've never broke 14 inches with most fish being around 10 inches.
Have fun and feel free (please!) to IM me when you get back as to your experience.