A player gets training for each minute played at a training position (up to 48) But he doesn't get the same amount of training for each minute. The 48th minute is more valuable than the 1st.
We know that 48 minutes gets 100% training and that 0 minutes gets 0% training, and that 36 minutes gets less than 75% (36/48) training. But we don't really don't know how much. Some suggest that there is a rather severe penalty, and 36 minutes might only get 50% training. It's my belief that the penalty is less, and 36 minutes might be worth something around 60-70%.
But if you are training guards, it's almost impossible to train 6 players. So it is still useful to have a 6th player to soak up whatever is left over, even if they only get 50% training for 36 minutes. But it might not make sense to pay a lot for a 6th guard trainee, or waste the training on a so-so guard, but rather use it to improve your younger forwards.
So let's say that your forward gets 30 minutes at SF and 36 minutes at SG in the scrimmage. If you train PG/SG he will get 36 training minutes. He will also have 66 minutes total time for purposes of maintaining game shape.
As far as training SF, I would consider perhaps training them as a guard one season, and a forward the next. This might mean picking up a younger PF for the 2nd season, or a younger SG for the first season.