(46=46 teeth in the chainring and 17=17 teeth in the cog. people used fixed at velodromes. and yes, to switch between fg and ss you just have to get a hub that has threading for both ss/fg and flip your rear wheel around.)

figure out what ratio you like on your nishiki right now. the fg will be a little more efficient, but try to stay in the gear inches which you are comfortable. and plz, make sure you're putting a cog on a hub that has compatibility for a lockring.