21 spd would be a 7-cog freewheel and a triple-chainring. I think you might have problems finding a shifter to handle this. If you were to go with a 7spd cassette, then you'd need to find a 7spd shifter with a 3spd for the front. This could be doable - but may take time.

What would be simplest if to go with friction-shifters. They don't care how many cogs in back, or if it's a freewheel or a cassette.