We, I mean the community, know a thing or two about training, and you should read this guide. Player will not get more from training, regardless of amount of positions and key skills he has.
It doesn't matter if a drill contains grey drills or it's a "clean" drill -- the player will gain quality with the same pace. Only level of the drill matters.
And it is never nice to train as many skills as possible. If there are more and less useful skills, it is a waste to train less useful for the cost of more useful.