you are correct in that a fixed rear cog and the front freewheel (however strange of an idea it was to make those...) will work like a regular freewheel and let you coast. However, IIRC the front freewheel system still had a rear freewheel and a regular threaded hub. That means you wont be able to put on a standard cog and lock ring (barring ghetto suicide hub). So from a reliability standpoint you would probably be better off using a single speed freewheel, since it wouldn't get spun off should the front freewheel snag or something of the like.

Either way will have the same end function, but unless you have a fixed hub, just use a freewheel in the back.