Ok so I went back to my LBS and they exchanged the 15T for a 16T, no sweat. Get home, cog threads right on; moment of rejoicing. Moment of glory quickly fades as I try to put the chain on. Chain doesn't fit. The teeth, upon closer inspection, are much wider than the fixed cog.
Well at least I'll have something to do tomorrow.
Thanks to everyone who helped, failure is really only prolonged success.
