Most of the times you should either focus on just guards or just big men. I think there's something to say to have 1 of the other as 3rd backup trainee (so 2 guards and 1 big), so you can play a trainee at his natural postiion when you do out of position training for your main trainees, so you can perform better in games. Normally if you want to train inside scoring for guards, you need to play them at center which isn't very effective. If you have a 3rd trainee that's a big, you can play him in a game you need to win. When training guard skills just play him in your scrimmage so he doesn't hurt you. This way you can build a nice backup SF of sorts with your backup guy.
Otherwise, just guards or just bigs.