I have not been to Yellowstone in quite a few years but the experience is beyond what is normally regular fly fishing in most states. There is a lot of water, different fishing opportunities, and most of it is the way it used to be before Industrialized America. Wild fish and insects that feed them are in abundance. It will be for most fisherman a life time experience and that's the way it should be. Sometimes the fish are so easy to catch you just have to look around and wonder about the fuss made over the sport's complexity. Add that the hatches and fishing conditions are predictable. You can depend on what the fish will eat for the most part on what time of the year it is and local fly shop reports. It's a cycle that's been going on forever.
For the most part if you can drift a fly for a couple of feet in a mixed current in a river you will be able to catch any fish the park has to offer and if you experience refusals change the size of your fly, most often this will solve the problem. Still water in the park has all the midges, callibaetis, damsels, dragons and scuds you would expect, so fish those. I think you will have the flies you need to fish the park. It's not really that big a deal as far as fly selection. Work on a few stream tactics and learn to cast a dead drift dry or wet fly when you need to and you'll be okay.
Hoppers are great in August in the west afternoon.
"As far down the river as he could see, the trout were rising, making circles on the surface of the water, as though it were starting to rain."- E.H., The Big Two Hearted River