It all depends on which rivers/streams you fish. If you hike in to an area like mora flats, or jacks creek, or any of the smaller creeks, 8' max. The pecos itself is never very big, I had a 9' rod when I was there and definitely wish I would've gone shorter. I think your choice is fine except it could be tough if it gets windy, maybe go with a 7.5'-8' 4wt. Good luck! that's a beautiful place to fish.
TCU class of 2019