Re: Marines and Combat Personel?
Anything's possible, the fact is that Trek has always focused on the heroes. It could well be that specific ground troops exist. We may even have seen them on-screen, but no character has ever said "Look, that's Colonel T'Vill of the 27th Infantry Division."
Colonel West in STVI is given a traditional army/marine rank rather than navy like most of Starfleet, which seems to indicate there are specific ground troops somewhere!
"A perfect organism, unclouded by remorse, or human illusions of morality." - Ash, Alien