Sacrilege - gearing up a fixed

hijacking the topic, it's perfectly fine to put a 5 speed block on an 130mm rear spacing? I'm building a bike up for my dad, and decided to have 1 chainring and 5 speed rear as he never change gear on the front.

If you can find a 5spd cassette that fits on a 130mm hub then yes it should work i think. However I think most 5/6spd stuff is all based on a screw on freewheel/cassette, so it might be hard to find a cassette suitable for a 130mm hub freehub