I know from the faq that the sequence of precombat effects (ex: assault cannons , anti-fighter barrage, action cards, etc) are determined by the defender. So does this mean I could use my dreadnought assault cannons to destroy my opponents destroyers before they can do their anti-fighter barrage if I am the defender? Or is all pre-combat damage simultaneous (meaning casualties still get to fire) as happens in normal space combat sequence?

So does this mean I could use my dreadnought assault cannons to destroy my opponents destroyers before they can do their anti-fighter barrage if I am the defender?

Yes, though to some degree that is up to your enemy as he decides how to apply the casulties from your assault cannons.

Right, assuming you blast through his fighters and he elects to take destroyers as casualties instead of other units.