Every position should have single training available with limitation like PG can't have inside skill single training.
SG - SF should have single training for OD PA etc...
PG - SF should have single training for JR
PF - SF should have single training for IS ID
PF should have single training for Block
etc
If you look at the training system a SF trainee has to play anything but SF to progress in single training, PF too . it makes no sense.
You want to train your SG ? he should play SG, you want your C to have some SG skill ? he should play SG.