You really should be looking at training a position, rather than a single skill, and personally I'd train both guards. I'd look at point guard as being a shooting guard with good passing, and a shooting guard as point guard with good shooting. They both need good outside defense. You can use a little variation, such as perhaps when training passing concentrating it in the PGs, and then when training shooting or outside defense give it to the guards. If you happen to have a good young small forward, you could even use him as a backup trainee.
It is pretty hard to train 3 players at a position in BB, but you can train a 3rd player some. So instead of a 3rd trainee, you might use one of your other young players at another position to take up the extra training.
I'm not sure about what I'd do about the age. Each season, every team gets 3 new draftees (18 and 19). This will happen around August 10th. So every team will be releasing 3 players, either some of their draft picks, or older players that are being displaced by draft picks. During the rest of the season, the only "new" players are players being released by new teams who realize they can't keep 18 players. These are more typically cast offs, particularly older players.
So I might look for a little older, in their 20s and then take another look after the draft when there should be some more young players. It could even be that you'll decide to train your draftees and switch training.