In most wargames you have a conflict between what you, the player with your omniscient view of the battlefield, what you want your troops to do and what your troops, with their limited view, would want to do. This is represented in some games by 'must engage closest enemy' rules or by rolling to see if your orders are obeyed, etc.
X Wing, in it's quest for simplicity, has bypassed all this in favour of simply having your ships do whatever you want. This results in suicidal martyr pilots who are willing to sacrifice their lives for the cause. You don't notice this so much in the 100/6 games because there's not so much going on, but in Epic you wind up with slightly odd situations where incoming fighters totally ignore their enemy counterparts, sacrificing their own lives in order to fire a torpedo salvo at the capital ships.
Which is a bit odd. Not enough that I don't enjoy the game, in fact, epic games at 300+ points are by far my favourite way to play the game! But the lack of a way for fighters to 'pin' their counterparts in place is notable in it's omission.
It's going to depend on the ship - TIE Bombers, Y-Wings and B-Wings would absolutely ignore enemy fighters to make a missile / torpedo run at a capital ship.
Perhaps a general rule that says something like "Small and large ships may not declare a huge ship the target of an attack if they have a small or large enemy ship within their firing arc" ( I don't like restricting it to Range 1 - you really don't want all your fighters bunched up close to your Epic ships) then have some kind of squadron designation card that reverses the rule for your ordnance carriers - so they can attack the capital ship but must ignore its escorts.
Or maybe make each player apply a squadron designation to each group of his ships that assigns them particular target priorities that must be adherred to:
Interceptor Squadron - must give target priority to Escorts, then Interceptors, then Capital Ships, then Bombers
Escort Squadron - target priority to Bombers, then Interceptors, then Escorts, the Capital ships
Bomber Squadron - target priority to Capital ships, then Escorts, then Interceptors then Bombers