It's quite an interesting discussion you've thrown up. I think if you look at programmes like the new Galactica, that seem to me to have one of the more believable representations of what life would be like on a working military space vessel, the whole structure closely resembles life on say, a present day air craft carrier. Think of engine rooms, living quarters, the galley, a bridge, all this is drawn from the Navy structure and has more in common with that than the Marine Corps or Army or Air force.
I think Star Trek makes that point really well, too. I think Picard has a ships wheel in his ready room or somewhere.
Which leads me to believe at the level of USCM ships like the sulaco I would maybe imagine a 'Navy' run vessel deploying Colonial marine troops – all though clearly it's not portrayed that way in the film.
So despite todays formula where you have Air Force personnel going on to NASA, which makes perfect sense, I would expect the Navy to have a substantial influence in the future when large military ships are a reality, dealing with deployment, fleet auxilary, etc.
Maybe someone who is attracted to life in the Navy in the early twenty first century would be attracted to joining the 'space fleet' in the twenty third century. Personally though, I think in the Aliens universe a Navy uniform is really stretching it!
But hey, there's no rules – go for it!