That would be my choice. Not a lot of the waters in the park have huge fish. I use a 3 weight for a lot of the waters. If you’re going in the very near future, make sure you check the closures due to heat. If nothing has changed (and I don’t think it has), everything is closed to fishing from 2 p.m. to 5 a.m.
If you fish outside the park, I’d still go with the 9-foot 5-weight. And still check for closures.
Last weekend I used my 7’6" 3wt on a very small creek. The next day, I used my 9’ 4wt on the Gibbon. I think a 5wt is as big as you need to go for the park. The fish were no larger than those on my home waters and the Gibbon doesn’t have the velocity.
If you have to bring only one rod, a 5wt is a decent choice. If however you have a 4wt, you won’t go wrong. Depends upon where you plan to fish.
If you only took one rod, then a 9’ 5 wt would be an excellent choice. I use either a 4 wt or a 6 wt since those the rod weights I have and use.
As of Sunday when I came back through the park, there were still a lot of restricitons. After checking the website just a few minutes ago, it looks like all of the restrictions have been lifted on all waters in the Park. Depending on when you go, I still might think about skipping the Firehole since it is probably a bit warmer than most due to the thermal runoff. The Madison is just a bit cooler as well as the Gibbon. You should still check the water temps when you go.