I'm sure someone else will jump in here but I would think 20A would be fine all the way around providing your turn signal wiring is the same gage, or close to the same gage, as the rest of your fused wiring.
The fuse's job is to protect the wiring in the event of a short. A short is infinite amps so your wiring and fuses don't care what the voltage is.... only the amps. If your wiring was designed for 20A in a 6V system it will do 20A in a 12V system as well.
Best regards,
Les
