Pass interference requires contact.  So “face guarding” is not a foul without contacting the receiver. 
As far as how aggressive, hard to answer without seeing a play. It seems one of the hardest things for the coaches to understand is that the defense has the same rights to the ball as the offense.  There can be fairly significant contact but if it’s an incidental result to both players making legitimate plays on the ball and there will be much more latitude given.  Also, once the defender has established his position, It becomes the offensive players responsibility to avoid contact with the defender, not the defenders responsibility to get out of the way.