If 6V electrical systems were "Great", then why did the auto manufacturers switch to 12V ? I had a number of cars with 6V systems in them years ago, and had trouble with each and every one when trying to start them when the weather got cold. And those cars were properly maintained. Never again...ever !!!
And to answer your original question, I jumpstarted my 1948 Plymouth (6V) from a 12V battery numerous times when it refused to start in cold temperatures. Usually started right up, and didn't seem to suffer any ill effects.