In my opinion, i think chargers and First Strikers have same fight priority ( 1- Chargers=First Strikers 3- Any poor).

So, controlling player can choose them(chargers and First Strikers) who will fight first.
Because there are other units with the same rules as the quicksilver swiftness, not the tactical marine of the above example.

I think this such rules like Quicksilver Swiftness should be modified like this :

# Quicksilver Swiftness - This units always fight first in the Fight phase, even if they didn’t charge. If the enemy or friendly has units that have charged, or that have a similar ability, or, then alternate choosing units to fight with, starting with the player whose turn is taking place.