I have an alternator with the stock ammeter, no problems, works as it should. My guess is that since the ammeter measures current, the voltage shouldn't matter (assuming the ammeter is designed to handle the expected current). Actually the 12 volt system probably produces less current than a 6 volt one anyway. And the alternator is only going to produce the current your system is designed to consume. I have a 100 amp alternator, I never show more than 30 amps on initial start-up (starter draw). After that it deflects back to mid-point as it completes charging.
don
