Flexibility in terms of what is excessive force, sure, that is the referee's job to determine what is excessive force.
Barton used excessive force when not challenging for the ball. He is guilty of violent conduct. He should have been sent off.
If Barton pushed him he'd be using 'excessive force' man
Force should technicaly be never used AT ALL in an off the ball incident, but going word for word by those rules we'd see players sent off for jostling in the box before a corner.
He hauled him up by his shirt, man. How many times do you see something like that? That's excessive force in my book.
Without doubt. Cowardly and malicious.