i would agree with the training of players for the nt as to hurting your own team could be a problem. I know I have focused alot of my teams training over the past couple of months toward a player that will be ready for the U21's in another month or two. But this has come at the expense of other players and positions for my team.
I figured I should do this, especially since I put my own team ahead of the nt previously when i stopped training my backup Center, who would have been good enough this season for the U21 team if i focused more training towards him during the past 4-5 months.