Cheap cogs 16T and under = Shimano Dura Ace.

I got a 17T Formula cog to start off with on mine, and that worked until I rode it into the ground and gave all the teeth a nice curve on it (chain wear through lack of maintenance... don't do that). It's also still stuck on the hub, clean the hell out of your hub threads and use grease when you install your cog and lockring. And use a chainwhip or rotafix to get it down tight before riding off.

Yes, you can re-use lockrings.