15T Freewheel Which Works on Flip Flop Hub
The normal freewheel thread is too large for cogs smaller than 16T. One source for the information is Sheldon Brown's singlespeed page. It is a good one to read before buying any more parts.

A BMX flip flop hub will, however, take smaller freewheels that have smaller thread. Fixed/free flip flop hubs have the larger freewheel thread and similar sized thread with lock ring thread on the other side, these do not work with small freewheels.