o yeah,
I forgot to say that a normal hub that takes a standard 16 tooth freewheel
can also thread on a fixie cog and a lock ring.
the smalles size you can use is 12 tooth.
pair that with a 24 to 26 tooth front sprocket and you should be good to go wheely around
and backwards also.
to take off a freewheel from a hub is pretty hard to do.
I have to usualy destroy the freewheel to get it off.
crush it in a big bench vise, so it wont spin anymore, and spin the rim off of it.
its a mega pain in the arse.
check your local bike shop for a crappy cheap wheel to try out a fixie cog on.
then if you realley like it, buy a good one.