Go with Tiny on this one Jud..
The 6 volt system requires TWICE the current capacity of a 12 volt system. This means the internal wire windings and everything related to that starter is twice as beefy as a 12v system. So don't worry AT ALL about the 6 volt starter. Its a GOOD thing. As Tiny says, don't put 12v on the starter for long periods of time because it creates excessive heat. But you wouldn't do that anyway. No impedance numbers, but just common sense, no worries at all.
