[Bongle]

My 700x23C (27in skinny racing-style rim) wheel with steel spokes weighs about 1kg. I assume a 20in one built much more robustly would weigh about the same, so weight shouldn't be an issue.

That said, I've seen one team run bicycle wheels and they didn't perform well. Things to consider
-Spokes are pretty delicate, and might get rammed by an opponent. Fixing spokes is a huge time-consuming pain.
-It'll be really hard to turn in a 4WD configuration, unless you're doing 2WD + caster.
-The torque problem - make sure you gear your motors waaaay down so they aren't tripping breakers when you accelerate or get into a pushing match.