Myy understanding of the slingshot effect is similar to the other response here. If a single player, such as a guard, has 'respectable' in all the guard skills such as JS, Handling, JR, Passing, but his Driving is atrocious, the slingshot effect works, such that his Driving skill will pop faster than all his other skills. If one skill is down when compared to other skills, it in theory should pop quicker than the higher skills.
its not so much as 'the higher skill, the slower the training'. This is incorrect to say it like this. I could have a guard, with Wonderous JS, JR, Driving, Handling, but only Respectable passing. The slingshot theory suggests that Passing should train faster than the other skills.
The skill level of other players has no effect on each other with training.
The other post here also has links to good info about training speeds of two position v one position, id check them out.
All in all, i think one of the 'traps' can be that you have these young trainee's and you get attached to them. Sometimes you have to be 'tough' on your players and sell them. Dont worry, you wont hurt their feelings.