Of course 1 on 1 give a lot of pops, because it trains 3 skills almost as fast as one position training. But out of those 3 skills, only one is primary and other two are secondary. I don't want to say DR and HA are not important, because they are. But since you are limited to only 3 seasons of training for U21, you have to spend that in best way possibile. If you train only JR, OD and PA for three seasons, DR and HA will rise each for 5-6 skills alone. Also JS will pop every three weeks of JR training. So I would advise to train 1 on 1 no more than 4 weeks alltogether in 3 seasons before reaching U21. Unless of course, you start with very low HA, DR or JS.
HA and DR are secondary skills because they "support" the main skills, mainly JR. DR is the ability for a player to make his own shot. But if JR (outside shot) or IS (layup) is low, the what good is high DR skill for? Same with HA: your player can handle the ball, well done. But if he has low PA or low JR, what should he do with it? From my experience, JR and JS for a guard should be as high as possible and DR and HA should not be trained to higher levels than JR and JS. Ideally HA and DR should be on equal level of JR (or PA for a PG) or maximum 1-2 levels lower. At least that's how I train my guards and for now, I find resulsts of this training efficient.