It is routine in flight planning to tailor the fuel load to match the mission. For ammunition, I can only speak for ground operations, but here it is routine to tailor the loadout to the mission as well. The "basic load" exists simply as a baseline for staff purposes - a standard, general purpose package that is modified to suit circumstances as required. My very limited exposure to strike planning suggests that load tailoring in the air force is even more specific, which makes sense given the relative scarcity and expense of both platforms and munitions.
So in general I think there should be no issue with tinkering with the loadout to the finest detail, up to the limits of the aircraft capacity (and certainly beyond the by-the-book limits of maximum takeoff weight - this is commonly mentioned in histories). The only possible exception is the point on drop tanks - I'm not aware of drop tanks being taken for any reason except range extension, which would imply full internal tanks, but it makes sense to do it for maneuverability purposes if they're available, so it may be that it was done historically. I know Spitfires on short-range ground attack missions in Normandy carried slipper tanks when it would seem full internal fuel would be enough. However it may be they were taking extra for insurance, rather than getting light for maneuverability (a secondary consideration in the ground attack role).
