Firstly, thats sell on clause is gonna kill you in a few seasons, you may as well keep him until he retires.
Anyway, -10 doesn't gaurantee a player will become world class, just that he has the potential to be. I've seen Aaron flop for me after 3 seasons.
Of the -10 players though I would partner Saivet and Vela, because in my experience once they hit their 20's both become goal machines every time. Never have Giovinvo or Anderson in my teams but I have Lulinha now and he is a legend, play him just behind the front two and you will have a lethal attack.