|Pick your launch platform.|
The Pinnace (at 40 tons) has enough interior space for it to carry a 10-ton AFV from orbit to the surface, or two G-Carriers, or five air/rafts. Marine armored cavalry delivered to the front lines. The Slow Pinnace has even more space, so it could load up 7-8 air/rafts, five speeders, or (tight fit) three G-Carriers.
Modular Cutters, with their very flexible 30-ton modules could transport mobile headquarters, medical or other buildings, supplies, fuel or artillery to the surface. Medical craft could provide care on the ground, or evacuate wounded in low berths back to the orbiting ships.
The Shuttle (3G accel) could be loaded up with anti-vehicle and anti-personnel weapons and become the Assault Shuttle, ready to bring heavy air support while transporting Marines. If each marine needs 1/2 ton of room for himself and his weapons, a company of 140 Marines could be aboard.
Single or dual place Fighters can be employed as air support for ground operations or to counter opposing air forces. Fighters would also fill the role of tank destroyer. Starship-grade weaponry should finish any ground vehicle with one good hit. I believe Striker says so in its section on aircraft.
Fighters or Ships' Boats (both 6G accel) could be manned by Marine pilots as screening elements, interceptors or attack squadrons. It would save a bit of crew space aboard if the ship's craft had Marine operators instead of Navy.
What else could Marine pilots do if we let Vehicle include the Ships' Boat skill?