Firstly, I remember seeing this sometime back, so you might be interested in it:
Emulating Pellegrini's Man City
For roles, I suggest to keep it simple. Whatever you would normally use for a team that doesn't have an array of highly-talented players can be equally used in a team that does. In your case, Manchester City. For example, your defence: full backs on Full Back (Automatic), and defenders on Central Defender (Defend).
For every one role that is better attributed to someone with a specific skill set, use two roles that can be operated in by players who don't require that. So for a 4-2-3-1 you would have AML - AMC - AMR. Both wingers can be on Inside Forward (Attack) whilst you can use the man in-behind as a Trequartista, Enganche, False Nine (etc) if you wish.
If you use a method like that -- for every one specialised role, you counter it with two simpler ones -- then it can maintain balance in the side. When tactical familiarity increases, then other players can be slotted in easier (e.g. substitutions for key players) without having to change the role and everything else.
If you're unsure by what I mean when I said 'specialised', or 'specialist', read
this great guide by wwfan on the SI forums.