No, this is not a 6-digit system. It is a 3-element 2-digit system. Yes, there is a big difference. The number of digits in a system refers to how many digits are encoded into a single element. Elements are combined into images and scenes.
More here:
Not at all. In this case I’d use my 3-digit system: aNTeaTer, NaTHaN, and NaughTyMeg. Or, if using a 1 or 2 digit PAO system, you can make person 2 interact with objects 11, 12 and 13, or person 21 interact with object 1, 2, and 3. This would be quite easy to recall properly. If you need to store each as independent items, using 3 consecutive loci makes it easier still.
For 4-digit years, you could designate a person for each century and an object for each decade. So 1925 is person 19 object 25. 615 is person 06 object 15. Tons of ways to slice this, but using a PAO list makes it simple however you want to tackle it.
Again, if you’ve already learned 100 things that you associate to 00-99, just learn the other 200 to fill up the full list. Its really not that much more at all. It will give you tons of non-repetitive combinations and variety to bring to your imagery. If not, you’ll be stuck with repetitive imagery and constantly be dealing with the problem of sameness.