my iro flip flop is a fixed-free hub, meaning one side has a reverse thread for a track lockring and the other side does not. this other side is designed for a bmx type freewheel. i wont tell you that you can't put a track cog on there, but you can't screw a track lockring on, and therefore many would consider it a bad idea. do a search for "suicide hub" if you want to know more.

the advantages of running a brake are never more evident than when you're relying on glue/friction to stop your bike.