I have read on this forum several times that there is no problem putting a freewheel on the fixed side of a cog but I do know that a track cog with a proper lock ring is better than a suicide setup on a normal freewheel threading. Because of this I have been told by my lbs and read many times that it makes sense to get a fixed/fixed hub. Fixed/fixed can do free/free, fixed/fixed, fixed/free. A flip flop is not as flexible.