A 6 volt starter will operate on 12 volts but eventually it will burn out. We get away with it for whatever amount of time because the starter is only used briefly each time. How long you can expect it to last depends on what condition it was in to begin with. I got 2 years of summer cruising from my original 37 Chevy 6 volt starter after I converted the car to 12 volts. On the plus side that baby will crank your engine over like you never would believe on 12 volts.
To answer your one wire question, the single hot wire from the Batt post on the back of the alternator can go directly to the battery but looks a lot neater if you solder an appropriate sized ring terminal onto it and attach it to the battery terminal of your starter solenoid or the start switch on top of the starter if you have the foot operated starter. Either way it ends up at the starter end of the battery cable. Make sure you use #10 gauge wire at the very least and if it is a high output alternator #8 gauge wire is better.
Mike
