6V mods typically refer to stacked 3V LiFePO4 batteries. Usually they are rcr123's but some use CR2's.
Commonly they are made by Tenergy, AW, and ultrafire. There are DX generics as well.
Most 18650 mods can be dual purposed to take 2 stacked Cr123's.
6V is likely to burn a 510 atty. I only use 6V on 901's and 801's.
The set up doesn't require a regulator just 2x 3V batteries (preferably paired).
Loaded voltage is not 6V, its closer to 5, but in some cases as high as 5.7V
5V mods refer to regulated mods.
You should always use a 3amp rated swtich, or a mosfet.