My biggest piece of advice is hire a lawyer!
Anyone who thinks their own state has confusing regs should look at Yellowstone's. Rivers seemingly change rules by the footstep. Seriously though in all honesty, talk to your guide and/or local tackle shops and get the most updated regs you can.
That said, September should be best time for the hopper/dropper. Size 6 or 8 I like for hopper, with your favorite nymph below it (prince, san juan worm after rain, pheasant tail are the standy's). Cast TIGHT to the banks. If you don't lose the dropper every once in a while you're not tight enough. The further away from roads you can get, the better. We as fishermen have dreams of fishing the mighty Yellowstone River in peace and solitude, catching big rainbows and cuts. Truth is the 'stone is a zoo as well as most other famed rivers like the Madison. I honestly didn't fish a whole lot in the park, mostly outside in MT. If you head out that way, let me know I can offer better advice...