Sorry for dealing with the game report first, I wanted to get it out of the way.
For this to happen I am thinking that You need a 18yr old or a great 19yrs. However, 19yr old's have less time training time. So if you have a 18yr old. Is it best to train one year ID, then second year IS and then third year RB? or would you level each one up a couple and then move on to the next one etc..
To answer very simply, in your words I'd level each one up, then move on to the next. However, its more complicated than that. For a start, I'd want an inside player to have SB as well as the other three skills. And at both u21 and NT levels players need more than just those primary skills. All players should have DR which should be at least close to the highest primary skills. Also, all players should have secondary skills. For bigs that's the guard skills, and vice versa.
If that seems vague, its because its difficult to give a more specific answer as it depends very much on the player's starting skills.
You are absolutely right that if you're talking about a 19yo he would need to be great compared to the 18yo.
However, PG.
Height - Up to 6'4" 18yo min. - Perennial Allstar Potential (7); OD+HA+PA>14; 30 Guard Skills
21yo min. - 14OD, 13PA, 65 Guard Skills
when you are wanting to train a PG If you trained OD and then PA does the JS and JR need to be high?
Again this can depend on the situation. For the NT certainly he'd need JS and JR. Personally as NT manager I'd accept a player with better PA if his JS and JR were lower but not everyone would. For the u21s it probably depends on the manager's taste too, but always given similar OD/PA, then the coach will always select the better shooter.
As I said above, guards with inside skills are also more likely to make the squad than those without.