Hey, stop the beers, I need someone to confirm that my plan makes sense :p
Obviously balance will be important. The easiest way to ensure things work out is to make an assumption of how many points you want a player to rise per rating in his career (about 600 was my guess in SS1, so if he started at b/av or above, he'd finish his career on bril) and then work out how many weeks he'll exist for (again, it's about 600). So you want about 1 point per rating per week. Less for his weak skills (about half a point) and maybe double for his strong skills. In SS2, it's basically the same except it's ~60,000 points over ~600 weeks, so about 100 points per week.
About half of that should probably come from playing in matches, which is important because those playing in matches should develop faster (other games do this through match experience, but it works just as well in my opinion to do it directly through ratings). I'm not sure if I did that in SS1 but I'm fairly sure I did. Youth players get bonus ratings but that doesn't matter, that just makes them better but shouldn't change the overall balance.
So you're down to about 50 points per rating per week from training. There's basic training which theoretically could cover about half of that, so a skills session would add about 5 points per rating, leaving about 25 from trainers. Since there are 4 skills trainers for outfield players, each should therefore add about 6 points. Probably easier to just say 1 point per skill rating, so if you have all 4 at bril, you'll be slightly ahead of the curve, but missing out on the other trainers. Of course they vary depending on position, so a bril Def trainer would add say 20 to tack and head, Mid would be 10 for each, Att 20 to cont and shoot.
I think those calculations are right, I'll have to double check them. It means that if you employ all 4 trainers (excluding the GK trainer who's separate) at max, continually, you'll be getting about 40 points per week per rating. Over 600 weeks that'd be 24,000, so about 2 1/2 full ratings from the trainers. Assume 1 1/5 ratings from training sessions (which are free) over 600 weeks and about 3 from matches (if he plays in every match, which isn't very likely), so you get 7 ratings, which is about right. With the new trainer-wage cap you're not likely to have maxed trainers so you'll probably get the 6 ratings I was aiming for.
Hmmm, probably didn't need to explain all of that but it helped me work it out in my head and someone can correct me if I'm wrong now