I like to have two or three formations, one defensive, one balanced, and one offensive. Changing formations is ok so long as your players do well in their spots, some dual role guys do better in one over the other.... even an ST or MC might do better on the left or right.

But... that being said, I just think you are at a high enough level that the difference in Q is less important between a four or five star, and so it's harder. Maybe try to leave them in a balanced formation for awhile, play friendlies if you can to try to figure where they have good form.