Arrows only affect the starting positions in the three mentality modes.
In the case of a MC he will move into a AMC position when in possession and move back to centre when you lose possession.
MC need to balanced IMO.
I played a team last night in the cup, he had butchered his players abilities.
2* ST with like 30/40% more on attack than the others, same with his his MC's defenders were also in the same mess.
While I think you can tweak them to a degree, go to far and you ruin the player for a few seasons or until you pass him on to someone else/mug who buys him.
I find the examples given in the 'justdomes guide to a manager' work well and I stick pretty close to them.