This is what I loosely meant but you described it way better! It's interesting that you mentioned a player fouling out due to personal fouls. I haven't really observed the definite trend myself that players would foul-out more often (or gather more fouls in general) against stronger opponents, though the logic is there. Therefore I always thought there to be some hidden balancing factor in the game engine that would avoid that.
But I agree that MrJ makes a good point that it always depends on what type of training you have active for the week and what type of players (age and importance in roster) you are training for choosing your scrimmage opponent.