You should not really be tinkering with each and every player's role within your tactic since alot of them will fit into the same mould or template of a role of one another regarding their position, therefore carry out a similar job. What you want to do is to create a tactic that, overall compliments another role or duty, with the strongest amount of reciprocation possible.
For example, two central midfielders in your starting eleven will have their own strengths in certain areas - i.e high Decisions and Passing attributes for one, high Marking and Tackling for the other - but if you select the right roles they can compliment each other greatly and obtain a much more rounded tactic.
The same goes for central defenders and central strikers. These are all bands of partnerships/relationships congregating into one tactic, whether it is attacking or defensive.
Those two central midfielders should, for generalisation purposes, be Deep-Lying Playmaker on Defend duty/Central Midfielder on Defend Duty and Advanced Playmaker on Support or Attack duty/Central Midfielder on Automatic or Attack duty. They both have almost completely different roles but both achieve one similar thing, ideally in something such as passing and distribution.