about Gaitan he doesnt have the attacking attributes for that role in my opinion. he would be best suited for the STCR role.
What I would describe the messi role in my tactic is someone who is a very very good dribbler, passer and can also finish.
Not easy lol.
about diego,id say if you put a low creative freedom on him, you would probably get the best out of him in my formation. The midfielders I had were xavi and iniesta ( best pairing in world football) and i wouldnt say they had brilliant seasons. their role in my team is supply the ball to the forwards and close down the opposing midfield. you dont essentially need very creative midfielders. you have to remember my tactic doesnt get the 70% possession like barca do in real life.
oh and feel free to tinker with the tactic. I did throughout the season in players like villa, iniesta, pedro and Dani alves.