If you are training PF/C the player gets credit for all his minutes at either position.
A player also gets credit for all minutes when determining whether he maintains his game shape or not. So let's say that you were training PF/C and had a player who had 36 minutes at PF and 14 at SF. He would get skill training based on his 36 minutes at PF - but his game shape would be based on all 50 minutes.
Stamina does not depend on playing in games. It used to, but that has been changed. All players will gradually lose stamina, but it appears that maybe one week of stamina training a season will maintain it once players get to a reasonable level.
You can either concentrate on a single position, or perhaps PG/SG or PF/C. If you train 2 positions, you can usually train 5 players per week. You might use a SF as a 5th or 6th trainee rather than having a 3rd string PF/C. And if you were training JS, you could train PF/SF and leave the C out of training that week, or maybe use your best C as the PF in the scrimmage.
Players don't get any training based on positions other than the positions you train that week. Some skills can be trained for all positions. More players get training, but each player gets less training.
Rotation training probably refers to training different skills, so you might train IS one week, ID the next, rebounding the 3rd, etc. You want players to have well rounded skills. You really don't have to do a pure rotation, but you should get a player trained in all his important skills.
You should have at least a couple of young players who can benefit from training at each position that you train. In your case, I would make a decision on training the 18 YO C or the 19 YO PG, and getting another young player to train along side him; otherwise you could be wasting a lot of your potential training.