The Holston sure has more than just two different versions of Caddis. You are without a doubt seeing larger sized Olive colored Caddis than the size 20s. Being able to look at what you see flying around on the stream and matching it is the ultimate in fly tying. The one thing that I would mention is that most of the bodies are a Caddis Green shade and the wings are the Olive color. Also, the flies will start showing up at Nances Ferry first this Spring and the hatch moves up river. If you tie the Usual, keep some handy because the Sulfurs start coming off shortly after the Caddis get into full swing. There is also a small PMD hatch up in the mid river section.
If you ever get up to Panther Creek State Park, hollar at me and I'll show you a nice peach orchard. I'll let you pick some tree ripened peaches that will take you back to younger days.