Generally, the air dam keeps air out from under the car, and the belly pan smooths the flow. They can coexist, but if the air dam is lower than the pan, it decreases the efficiency.
If the belly pan extends forward of the air dam, that's a diverter which adds downforce in front.
Partial skirts won't help as much as lowering because they won't affect frontal area. Hangar door brush is a continuous brush available in 2" increments for length and would make the 'spray guards' you see on big trucks.
The front wheelwell has more drag than the rear but there is opportunity for an air curtain. See the
new Ford trucks.
If you want diamond plate, that's what you should have.