i would be weary of shifty cogs (cogs take from a freewheel that have shfting elements on the teeth) on the back.

i have had really bad wrecks on the street when i was hammering along and all of a sudden my chain just "shifted" right off the rear cog, next thing i new i was picking myself up off the street.

never had that problem with a singlespeed specfic rear cog.

but it is a cool idea and so long as your not hammering in traffic your setup could work just fine for you!