I'd actually specifically exclude long range sensors because of the existence of scanning installations.
if you go into depth with PDC's I'd include something on Orbital, or even deep space bases as well, that should be relatively easy though, build a ship without engines like Starfire!
but that would make a difference on ground and orbital slots as well, I think if you start doing rules for customizing PDC's it will grow pretty quickly, for example a PDC with Troop bays = Barracks, a PDC with Long Range Sensors = Scanning Installation
on a very loosely related tangent, why do Intelligence Agencies consume so many slots? like 1 more than a Fleet Command Centre