A kill streak kit will work with any version of that weaon. For example, you can apply a Grenade Launcher kit to a stock GL, strange GL, festive GL, strange festive GL, and so on.
Only weapons with kits will contribute to your streak. If my scout Loadout was a killstreak scattergun, stock pistol, and killstreak sandman, kills with the scattergun and sandman would count, but pistol kills would not increase the streak count.
Aight, thanks. Considering sticking some money on steam to pick a few up.