As I understand it, the "flies only" section is from the gas pipeline behind Kalama River Bait and Tackle upstream to the confluence of Hatchery Creek. That's just upstream of the parking lot near Beginners Hole.

An 8 weight fly rod with a sink tip is just about perfect for coho. But if you tie into a fall Chinook, forget it. You'll need a 12-weight to stop them. But there aren't many Chinooks since the WDFW traps them downstream and trucks them to the upper hatchery. I use my 9 weight with sinking line and a 7 foot tapered leader (10lb). Yarn flies in orange, red, green or blue seem to work, particularly early in the morning.

The best place to cast is at Beginner's Hole from the opposite side from Kalama River Road. With chest waders you can acess from either upstream or downstream. The coho stack up in Beginners Hole thicker than I have ever seen anywhere outside of Alaska. But be aware, the hole is very deep (over 10 feet) and the fish are flat on the bottom. It's not easy getting a fly down. Good luck.