Bole_ has posted a very good training guide in the forums, how to get 7 stars in training. I really didn't know some of his tips.
Thanks mate!

So, here is my go at it. I try to be more simple and specific, including recommended coaches. All in all, you are going to need 9 coaches, 1 of them could be your assistant manager (maybe for the hardest training part, which is Set Pieces). So, choose them wisely and remember that you can use the same coaches for your youth squad (no need to buy more there).

Note: Please do not expect to find all the recommended staff I include in this blog post, because your game may not have some of them. It is normal. But hey, you now have a guide to pick the best staff available on your quest to achieve 7 stars in all training categories.

Training Categories

Strength
You need to get 1 Fitness Coach who has 18-20 at "Fitness" Coaching statistic. And then assign him only to Strength.
A recommended and reasonably cheap candidate is Michael Brouwers.

Aerobic
You need to get 1 Fitness Coach who has 18-20 at "Fitness" Coaching statistic. And then assign him only to Aerobic.
A recommended and reasonably cheap candidate is Christian Jahan.

Goalkeeping
You need to get 1 Goalkeeping Coach who has 18-20 at "Coaching Goalkeepers" Coaching statistic. And then assign him only to Goalkeeping, you cannot assign him anywhere else anyway.
A recommended and reasonably cheap candidate is Arpe Boerma.

Tactics
You need to get 1 Coach who has 18-20 at "Tactical" Coaching statistic. And then assign him only to Tactics.
A recommended and reasonably cheap candidate is Luc Rabat.

Ball Control
You need to get 1 Coach who has 18-20 at "Technique" Coaching statistic and 15-20 at "Mental" Coaching statistic. And then assign him only to Ball Control.
A recommended and reasonably cheap candidate is Atanas Kikyov.

Defending
You need to get 1 Coach who has 18-20 at "Defending" Coaching statistic and 15-20 at "Tactical" Coaching statistic. And then assign him only to Defending.
A recommended and reasonably cheap candidate is Guido Vergeylen.

Attacking
You need to get 1 Coach who has 18-20 at "Attacking" Coaching statistic. And then assign him only to Attacking.
A recommended and reasonably cheap candidate is Olivier Dall'Oglio.

Shooting
You need to get 1 Coach who has 18-20 at "Technique" Coaching statistic and 15-20 at "Tactical" Coaching statistic. And then assign him only to Shooting.
A recommended and reasonably cheap candidate is Michel Albaladejo.

Set Pieces
Probably the hardest category to get 7 stars. You need to get 1 Coach who has 18-20 at "Technique" Coaching statistic and 15-20 at "Mental" Coaching statistic and 15-20 at "Attacking" Coaching statistic. And then assign him only to Set Pieces.
A recommended and reasonably cheap candidate is Diolei Candido.

Have fun reaching 7 stars in all your training categories