I've been working on this for a year or so as well, and I couldn't find a single good resource...here's what I think I've got down (still learning):
The line(s) you want depend mostly on what you want at the end of them.
Your first question should be, is this a dedicated swinging line? If yes, is it for high water, or low water. High/fast -> Skagit (this includes big or heavy flies), Low/Slow -> Scandi (also for swinging high in the water column with light/small flies).
If No, then it gets tougher. The hardest combo is wanting to nymph with an indicator but also swing flies deep in high water. Skagit heads aren't great for nymphing, and it isn't just that you can't mend well beyond the head...they also don't drift as well as thinner lines. You can get multiple heads and switch them out (Skagit for swinging, a Scandi for nymphing), or go with a "switch" style line, like the switch chucker. On a 7wt the SC line can cast level T-11 (this is MUCH different from casting an "equivalent" sink rate polyleader, which will have a heavy butt section, but the tip will sink much more slowly than a MOW tip, which does so uniformly), and also works acceptably well for nymphing. It isn't perfect for either, but it works for both, especially on the GL tribs where you are unlikely to be making really long casts even when swinging.
Last, but not least, if you think you will only be swinging...definitely ask yourself if you mind not catching fish on a trip. At all. I've spent the last several months mostly swinging (first on the D, more recently on the SR). For days where I solely swung flies, 1-3 fish seemed a normal day on the D, I got one follow from a steelhead in 2 trips to the SR swinging only. In comparison, I hooked 2 fish nymphing with egg patterns on the SR in one morning. I'm not the greatest steelheader using any method (though still working on it!), but swinging is definitely not a numbers method from my experience. On the other hand, the take is addicting.
