Over the many years I've been fly fishing, I've been told frequently enough "that's not going to work here". Sometimes that information was correct, many times it wasn't.
IMO, you're on the right track by observing for yourself. I like tying & fishing stoneflies, and have used some rather large sizes & had success, even after being told, it won't work. I've spent more time pursuing bass than trout, but both will readily eat them.
The fact is, even though you may tie a stonefly pattern in the many sizes & colors that you're observing, it doesn't necessarily mean when a fish takes your fly, it's because YOU intended it to be a stonefly. Fish don't read fly pattern books. A golden stone pattern could just as well be taken as a drowned hopper. There's no way of really knowing why a fish takes our fly.
I've told a story before of catching 4 different species of fish in a stream,(rainbow, brown, SM bass, LM bass) on the same day & with the same fly. The fly was a black stonefly nymph of my own design, and rather large for that stream, a size 6. I have no idea whether or not each fish took it as a stonefly or even if there were any stoneflies in that stream, much less that size. In reality, there were also large crickets in the grass along the banks, and plenty of small crayfish in the stream, plus it's possible there could have been hellgrammites in that stream as well. Any of which that fly might have imitated. The only thing I'm certain about is it worked that day.
Confidence in what you're tying on the end of your tippet certainly plays a big role in success or failure. I believe when we're confident, we concentrate better, and may fish a fly a little differently, in a positive manner, than when we're not confident.
I know very little about entomology. Because I've been fly fishing a long time, I can generally identify some aquatic insects, and the size relative to tying an imitation. I've had success & failure, and it's all taught me that nothing in this sport is always true. If you tie patterns that generally imitate what you see, and there are an abundance of the real thing, then chances are you'll do well with your imitations.
BTW, I hate tying with biots! They're stiff & IMO offer nothing positive to a fly. Any stoneflies I tie, I use rubber or silicone skirt material for the tails & legs and it has worked just fine.
What I've posted here has worked well for me. It's not the only way, and others will certainly offer their views.