Re: how much "modifcation" is too much to a classic trout fly?
Personally, I feel the only time you have to be careful when making changes is when tying dry flies, as you may not end up with something that really mimics the natural or even floats correctly. But, with nymphs, wet flies & streamers, you can do about anything you want. I hate tying with biots, always have, and never liked how they're so stiff. So, whenever I tie patterns that call for them, I usually substitute rubber or silicone leg material instead. Still looks similar, and for me has worked just fine. I also like the Flashback version of a Prince.
Remember, no one likes to be behind the big truck, but that's better than being under it!