In the 732 the wing pumps are in dry bays in the bottom of the wing, thus allowing for their removal without defuelling the wing tanks. The centre tank pumps are in the leading edge of the wing, for the same reason. Has this changed in the NG?
The min fuel requirement for running the centre tank pumps is, I suspect, to avoid the possibility of the tank being run dry and the pump overheating - the suspected cause of the explosions in the centre tank on the ground. In the air, one is encouraged to turn the pumps OFF as soon as the low pressure light illuminates.