I've worn a marmot precip on many of my Shasta summits. On summit day it will be windy and cold, because you'll be getting a pre-dawn start. Wind protection is what is most important IMO. Underneath I usually wear a marmot driclime windshirt and a base layer. As the day warms up you can adjust as needed.
OR crocodiles are good gaiters. I personally like the Black Diamond mercury mitts.
I prefer wearing soft shell pants - the Mammut Champ's work for me, plus a base layer. You aren't going to be glissading unless the snow is soft anyway. If it's hard you will be downclimbing. I wouldn't bother wearing hard shell pants unless it's actually raining. I usually bring along a pair of marmot precip full-zip pants just in case, and they usually stay in my pack.
One thing that I consider mandatory for Shasta is a balaclava, preferably windstopper. See above about 'windy & cold'.
I would bring a 15 or 20 degree sleeping bag. Definitely bring a puffy jacket for mornings and evenings in camp, and for rest stops.
The thing about rain gear is that if it's raining you are not going to be climbing -- at least if you have any sense
You will more than likely be packing up camp getting ready to descend.