Can I use an old cog for my new hub? I have an old wheelset with a 16T cog. It's currently a 120mm flipflop hub, and I'm going to be replacing it with a 120mm coaster. The coaster hub that I want doesn't come with a 16T cog (only 14, 15 and 17 for some reason). 

Rather than mess around with chain tension, etc, I was wondering if I could just use the cog from the old wheelset on the new hub, or if it's more complicated than that.

More info: old hub is a 32H, 14G, 16T cog. New hub is a velosteel 32H http://www.elegantwheels.net/STORE.html
To quote the page you linked, "Since the Velosteel hub has standard track threads, it can run any fixie/track cog built to Phil Wood/Campagnolo standards." For cogs, this is the same standard as every other cog, but the lock ring will probably be different. Just take the cog from your old hub and keep the lock ring from new one.

Note that this is different from most other coaster brake hubs, the more common Shimano, Torpedo and others use splined cogs.