I commute on a FG road bike, but I live in a very flat city. I have a SS freewheel, but have used it very rarely. I like FG for winter because of the instant feedback on wet streets and because it does force you to pedal the entire ride.

As for knees, I notice that mine are very sensitive to proper saddle height. Whenever I've changed saddles/seatpost/pedals, I could tell within a few mm whether I was too high or too low. Also, going with a lower gear/ spinning a higher cadence can help.

If you have a gear in mind from your multi-gear bike, I'd suggest going one cog more (that is, easier), especially in winter.