I believe that there is only one promotion system and I have found this to be true for generals, inquisitors, and assassins. The promotion system goes like this: from no stars to one star takes one victory as does the progress from one star to two; the number of victories needed to progress then grows exponentially, i.e., two victories for three stars, four victories for four stars, eight for five stars, etc. I actually trained a general from zero stars to six stars, and an assassin and an inquisitor from zero to four stars to test this. I have never trained an admiral to four stars (as they die off more easily), but his progress to three followed the above pattern.

I cannot speak for Caravel, but in the test that I ran in the earlier thread you linked to, stacks had virtually no advantage over single ships in an attack. However, an entire stack of ships can be sunk by an AI stack, or even a single AI ship. For this reason, I don’t see an advantage in stacking.

Run some tests yourself.