The Seuthe units that I use for my R/C tugboat starts at 6V. I know they have (at least) 6V and 12V models. Not that high of a voltage. Now current is another story! Mine draws 500 mA.
Yes, but the units installed in N scale DC models are usually rated for 12V (because the manufacturer does not want the smoke generator to go "poof"" permanently when some
adult kid runs their models at full throttle. A 6V unit would not last very long under 12V.
Same with DCC models - you wouldn't use a 6V unit there either.
Sure, they will produce smoke (at reduced volume) under lower voltages. And since these are simple heaters, they need lots of power (current) to heat up. If yours needs 0.5A at 12V then it is actually a 6W heating element. That is quite a bit of power.