Originally Posted by
Valyar
Quick Draw perk allows drawing a single weapon per turn as free action instead of minor action. Therefore, it does not give you any advantage in the combat.
In your example, if the Raiders will open the combat, as per the rules - the one who initiates the combat acts immediately. Then you arrange the combatants as per their initiative. This is on page 24. If you think the raider and your PCs are eyeing each other out and both know something will happen and it is matter of who will be quicker, then you can make a struggle (opposed check) between them and whoever wins - starts the combat.
Same approach when you want an ambush. Depending on the setup, can be passive or opposed checks.