Ok, I have an Iwana for fishing the Great Smokey Mountains National Park. What rod choice would you suggest for fishing in Yellowstone?

Depends on where and how you want to fish. The iconic rivers like the Firehole and Madison are Amago territory, IMO. Occasional big fish and strong winds. I like to fish the smaller side streams back in the woods, and those would be great for a smaller rod, though many of the streams aren't super brushy and can be fished with a longer rod.

I was thinking some of the backcountry areas near the West Yellowstone entrance. Fan, Gneiss or the Gallatin River in that area.

Good choice. The headwaters of the Firehole are also excellent.

Your Iwana will do just fine in either.

it is an 11ft, do you think that would matter at all?

I've not fishing that rod, but my understanding is that power to play bigger fish and reach are the main limits, perhaps casting into the wind as well.

All the streams I've mentioned are darn small, and I can't see the shorter rod being a major limitation. Your chances of catching a fish bigger than 12-14" is also very slim.

On the other hand, they're open enough to use an Amago (I've done it), so if you really "need" another rod than this would be a good excuse. Then you'd be able to use the bigger rod on the lower Firehole, upper Madison, and Gibbon.

I use my Amago for everything these days. Nice when you randomly catch a big fish in a tiny little steep stream. The other weekend I was glad I had the big rod, endless overhead snags nonwithstanding, when I hooked a 16" bull trout and it took off downstream into a series of 2' boulder cascades.