I distinctly remember being an emerging fly tyer. I used to just dream about what I could do with materials. Any anytime I start to tie something completely different from what I had been, I have a similar feeling. When I got into bass fly tying, I went NUTS!
So you have all these differently 'tweaked' flies; differently applied materials or different mixes than what you've seen. With modern synthetics, there are a BAJILLION things you can do. Now you have to fish them. Why fish one style over another? How to choose?
What you'll probably end up doing, if you're anything like most people, is that you'll fall into making your own version of a few different patterns with a tweak here or there. You'll find certain types that established fly tyers and fishermen tie and use. You'll make your own 'bargain' with things like body, techniques, hooks, ribbing, flash, colors and TIME. You'll possibly develop certain flies specific to certain bugs, like I do. Other fly tyers have just one style that they adapt in size and color; one fur nymph or only pheast tails. They follow what great flyfishermen like Left said. Almost any well presented nymph will work. (stick to a size similar to the bug.)
I have one stone fly pattern, 2 beatis (just dubbed and pheasant tail), 1 sulfur, 2 caddis (one's just a wet fly) and 1 march brown/ gray fox style. Then I have a version of a G. R. H.E. with a brass bead sz. 12-16 that's just kind of a 'whatever' nymph. Then there is a whole other world of attractors, egg/spawn, inchworms...whatever. For me, I need to have some reason to fish which nymph and I ties flies for those situations. There is a range of approaches. Maybe having whatever you made on a hook drifting in the water is OK. That's good because it clears your conscients of conflicts or discrepancies and you just fish.
Syl