The cogs should be the same size IMO. It is possible run a 1/8 chain on 3/32 stuff but it's better to run 1/8 or 3/32 throughout (chainring, cogs, chain).
Since you're going fixie, I'd say stick with 1/8 throughout. Just throw a 1/8 fixed cog on there, and make sure you use a 1/8 chainring and chain