Many advocate a 6 volt starter installed when converting to 12 volt. While I do not think it is a good thing long term, putting 12 V on a 6V starter will (should ??) not hurt it if done for 15 sec or less.

On a stuck and need to jump to get home I see no problem
EXCEPT paralleling the batteries with only jumpers. There will be arcing - I would make the final connection on the low (I assume 6 V - uhh why would you jump from a dead 12 V) battery ie the 6V. It would be best to have a relay make the connection but I have yet to see a relay in jumper cables and who really carries a spare relay?

Dan Bentler