To train who where?
the rating is depending on the salary of the player, and it is easier to raise the salary of the c then the one of the guard, thats why i think it would be easier for big guys then for small guys to reach the target.
What's the difference between star rating and grade rating?
the grade rating is a bit more detailed, because it could have the plus and minus. But a 5 star player would always have a A grade, but you could see if he is at the top end of the A rating through the plus or close to the 4 star rating when he have an A-