Got my Pro/Carbine yesterday (from DCB) and after a bit of cleanup it's starting to look pretty good

The only issue so far was the small o-ring in between the valve body and hardline adapter - after I disassembled it, it was so chewed up that I'm amazed that it wasn't leaking like crazy when I test-fired it in the backyard. Luckily I was able to find a substitute, which should do while waiting for a few o-ring kits to come in the mail.
A few questions though:
Will the velocity spring kit for the A5/98 work with the Pro/Carbine?
What's a good way to remove rust? I went at the screw heads with Eezox (a gun cleaner) and a wire brush, but the threads themselves are still showing some discoloration. I'm tempted to try the Diet Coke method

Do they make something that will slide over the sight rail and keep the linkage rod engaged, in lieu of the rear sight?
CO2 or HPA? My 9oz seems a little long on the bottomline, so I'm leaning towards using a 13/3000. I plan on using one of the older 50rd Brass Eagle Ammo Box pocket hoppers, so as long as I get at least 100 shots, I'm happy.
My stock barrel is 8.5" - is this normal? I thought they were a tad longer - maybe 10-11".