As the Ground Defenses structure requires marines as commodity, doesn't that already take into account marines defending the colony? Or do you mean giving marines to a colony before Ground Defenses are built increasing its defense value? In that case, maybe having Heavy Weapons worked into it as well would make sense and give some value to this commodity.
i.e.
Marines on their own only add a very small defense value to a colony without a Ground Defenses structure, with a soft cap/diminishing return.
Heavy armaments on their own add no defense value.
Marines+Heavy armaments (up to a proportion of say, 10 marines per "heavy armament") add a decent defense value to a colony without a Ground Defenses structure, with a soft cap/diminishing return.
Total defense increase can never reach what a proper Ground Defenses structure gives, once that's built marines/heavy armaments no longer add to defense value (they'll just be consumed by that as commodities as normal if shortages happen).
That something like what you may be thinking?