I don't know how much help I can be, given that I'm not too experienced a baker, but I'll try my best!
As far as the minimum amount goes, I would say (and I haven't actually tested it, I'm just guessing from my experience with 600g bread dough vs 1kg pizza dough), probably around 800g. It might also depend on the flours used etc...
I have only used it for up to around 1.5kg, I think it would be able to take more, but I can't say for sure. Smallest, I would not try to go under 500g using the dough hook. Using the mixer or whisk, you can go lower but I suppose that would be more appropriate for batters rather than doughs.
For bread, I mix on speed 1 for about 2 minutes to combine flour and water, then autolyse, add yeast and salt and mix another 3-4 minutes on speed 1, then increase to speed 2 for another 3-4 minutes, although I don't do the windowpane test as it never really works for me. For the bread dough, because I do small quantities, I have to stand next to the mixer with a spatula/wooden spoon and push the dough back toward the middle frequently. I'm not sure how this affects the kneading process, but I assume it makes it less efficient, which is why it takes mixing at higher speed to achieve similar results to my pizza dough, which I simply mix on speed 1 for 10 minutes.
I'm not sure what the kitchenaid disc is like, but there is a plastic cap-like thing which prevents the dough from getting inside the machine's mechanism, and at the same time works to deflect the dough. It doesn't actively push the dough down, it just stops it going any further up so any dough that continues climbing will push the dough that has already reached the top outwards, and then it will fall down under its own weight.
For low hydration dough, the strain is higher on the mixer, you can hear the difference. I made really high hydration ciabattas once, which required mixing on speed 10 for over 10 minutes (can't remember exactly how long), that definitely put less strain per revolution on the motor than when I make 60% hydration bread/pizza dough. I'm afraid I'm not experienced enough to tell the difference in quality between when I mix a high hydration dough and a low hydration dough. Mixing time doesn't really seem to be affected much. I used to make pizza dough with over 75% hydration, now I do ~60% and I haven't changed my mixing times or speeds at all.