If you're running 20v to a small fan, which probably pulls no more than 300mA (most likely ~200mA), you could use a
12v regulator. Power dissipation will only be 2.4W or less. It would need a heatsink though.
Using the 5v fan is fine on 6v though. If you were really concerned about overvolting it, just run a diode in series. The 0.6v-0.7v drop will bring the voltage even closer to 5v.