yeah, you're on the right track. concerning the freewheel, if you have one with the cassette/freewheel mounted on treads, the cog can be directly mounted to the hub. but if you have cassette mounted on a freewheel with splines, you'll need to rig the freewheel so it doesn't "freewheel" any more. i did this recently, but you'll need a metal grinder and a welder. i disassembled the hub, took just the freewheel, ground into it until i reached the ratcheting mechanism (the paws) and then welded the ground areas shut. i then got a cheap cog, mounted that on the freewheel and welded to two together. this is rock solid way to make a ghetto fixed wheel. for maximum strength, jb weld the freewheel to the hub! yay, ghetto fixie!