To make really good players you have to train them with one position training. But in your case I think it wouldn't hurt if you just continue with training all positions until the end of the season.
About the draft. Different people have different opinions. To me age is the most important thing. Then comes potential, and last will be grade rating.
So, if you want to make a U21 player, he should be 18 and have at least allstar potential. But you can still grow good players that are already 19 or have lower potential.
Depending what position you plan to train you should also consider the players hight.
Or just take the best you can get and sell him on the TL and buy a new trainee yourself on the TL.