You want either a group of young small fellas or big fellas but not a mix. You got bunch of young guards, so it'll be obvious for you to focus your training on the small fellas. You should train them in guard primary skills every week, on occasion people might do something different like train big men for one week or train the guards in rebounds but to start off just train them in guard primary skills until you get better knowledge on how to train.
Defense is always the best to train, anytime you see a team doing worse than another team although they got higher salary players, it's because that team is usually skilled more offensively and not defensively while the other is skilled defensively. I think the importance of defense is still understated. It is also very efficient way to improve your players because while you attack half the times and defend half the times, for small guys needing Outside Defense is only 1 skill where JS,JR,HA,DR,PA are 5 skills. Defense is by far the best way to win, but you don't want to be one dimensional, if you look at the market you'll see whats good like combination of skills to have if you want to know whats a good combo. It's best to be good in most areas at least. You want some Outside Shooting and Handling training as well in addition to defense.
In order to be effective in training you really want to train guys that can really gain worthy from training. Anyone below starter potential is practically garbage so you should forget about the backup potential players even if they're 18. You don't really have someone that good for training but if anyone then Applegate & McKinnon you should train but still you see small progress cause they're age 20 & 22. Training 1 player or 2 player training is best and debatable. I go with whatever is handed to me as number of players worthy for training, I go with 1-player training if I have a way to only squeeze for 1 position with my only worthy players to train. But if I got few worthy where 1-player training won't fit them all, I go with 2 players training. But again they really got to be worthwhile for me to train. I think there's mix opinions around so this is just mine.
The games per week part only refers to the minutes your player gets. If a player plays a certain position being trained, the number of minutes he plays in that position accounts for his training, you can go up to 48 minutes per player. The weekly stats will show you the minutes the players got in each position played.
Also note that the shorter the guy the faster he'll train in guard primary skills but the much slower he'll train in big men skills and vice versa with the big fellas.