well, you only have to start 3 players in a scrimmage, so with that, and your league games, you should be fine to get your main starters, who aren't being trained, 50-65 min per week. Usually I'd just start your starting post guys in both league games, and not play them in the scrimmage. There's no benefit to them playing in a scrimmage, and they can get hurt.
To single position train, any player who gets minutes at that one position will receive training. But remember that to get the full benefit of training a guy needs to have 48 or more minutes combined from the minutes that week. Since there are usually 3 games per week, at a maximum you can get three players 48+ minutes in the training position. To do this you'll need to have each guy play the entire game in the training position.
Say for example you want to single position train OD, which requires playing at PG. You have trainees A,B,C:
You might set up your lineups something like this
League Game 1 -> PG: Trainee A in all three spot of depth chart (starter, backup, reserve)
League Game 2 -> PG: Trainee B in all three spot of depth chart (starter, backup, reserve)
Scrimamge -> PG: Trainee C
In these cases, the trainees who aren't starting the full game, should be put as backups in other positions, so they can get a total of 60+ minutes per week, to maximize gameshape.
(note, if you are gonna single position, you'll want to make sure that you only dress 9 players in your game lineup, have your trainee at all three depth chart options, and it also helps to ensure that the reserves of the other four, non-trained positions are set with some player (usually the backup at that position). THis helps prevent your trainee from getting subbed out by the coach. Usually the only reason he won't get all 48 minutes is because someone gets hurt or fouls out. Then all bets are off.
Now, you could always train OD using two-position training, which you can do at PG/SG. Any player with minutes at either PG and/or SG gets trained. You could have up to 6 players trained like that on a regular 3 game week of course. But two position training is slower, about 70% of single position. So most people are single position training, to maximize how good their trainees can get.
That help?