I disagree with you about being able to any fly if you can tie a trout fly. I tied trout flies for 25 years before I tied a full-dressed featherwing Atlantic Salmon fly and it looked (still does because my wife insisted on keeping it so that I don't get puffed up) pretty bad despite taking me 3 hours to tie it.
Properly tieing a full-dressed featherwing Atlantic Salmon fly (not a spey fly, dee fly, or simple strip wing like a blue charm) takes a lot more skill than tying a trout fly, including those tiny little #24's. Classic New England featherwing streamers also take a lot more skill than a trout fly. If you doubt this, try tying a full-dressed feather wing or classic New England feather wing streamer and have it both look right and stay together when fished.