I'm not really sure how this should be handled, because I don't think a unit type really fits for a character, but I think it's fair for it to have it's own page, as well as the actual concept of a Marine (which one could easily argue that the Starcraft Marines don't technically even belong in). I'd like to write up a proper page for what the concept of "Marine" is but I'm not sure how to approach that. But on top of that, I think that it's cool that there's such a detailed page for the Marine unit in Starcraft, and trying to keep all of that information on with all of the rest of the potential content seems like a dangerous way to go, because there's a LOT of stuff on that page already. So, what to do?
I don't see any real problem with creating a "Marine" concept page separate from the existing USMC page given that marines do exist outside of the United States. Furthermore, compiling information about all of the various fictional and non-fictional marine organizations depicted in video games and comparing them to one another could make for a really good article.