I am putting together a couple of lists for to possibly play in a Quick Build tournament. One thing I am thinking of is using Kath Scarlet with the Marauder for the Firespray Patrol Craft. The card says to add a Tail Gunner slot but the quick build card only mentions Marauder. Would it be legal to add a Veteran Tail Gunner to Kath Scarlet?

If you add a Tail Gunner to the quick build then it is no longer that quick build. The Marauder title adds a gunner slot, but it does not equip any upgrade in that slot.

Note that marauder gives a reroll so it's still useful.

But there's no list building stage or points with quick builds so no opportunity to spend the points you don't have to fill the slot.