Think instead on teaching your young players a well rounded set of skills including playing in the post, from the wing, handling the ball out front, and penetration skills on offense to give them the best chance possible to eventually find the position as a grown player best suited to their skills. The focus should be on polishing those skills and automating them to the point they can execute them at all levels under pressure. I think that you should spend 15-20 minutes on these basic position skills per practice maybe splitting them into two different blocks around full court drills to space out the energy expenditure from high to moderate. Plus embedding your individual skills practices directly following a period of high exertion is more realistic to what happens in game situations as well where players will have heavy legs. Getting them comfortable with practicing their individual skills under these conditions will go a long way to performing them well during late game situations where fatigue can play a key factor in execution.
Fitness training is a constant for basketball teams.
You must work off season and in periods where you have no access to practice time in the gym to build a sound aerobic base which lets you players avoid a late season physical slump as season long fatigue sets in. Failure to build this aerobic base through running, swimming, biking, etc almost year round will show up in the springtime as your team fitness may well decide how you perform down the stretch run to championships. If you are working on offensive breakdown drills, then you will more than likely be working more on anaerobic capacity through powerful penetration and jump shot, or post move type drills with high intensity and high repetition. When you go to fast break or full court defense or press drills, you will be working more on aerobic capacity. As I previously said I don't think you need to worry about separating your fitness training for different positions because I am a believer in the all players having skills and understanding to play all position coaching school of thought.