Holy googly moogly, Batman! My new character sense is tingling.
As Wes said, welcome to Star Army. I'm Sigma, the GM for 4th Fleet. Please PM me about how you foresee playing this character and what your expectations for both the character and 4th Fleet are.
Now, unless you're planning to switch services (which probably has problems all of its own), I don't see a way for you to gain permanent command of a warship as a Marine. The NSN and NSMC are two different branches.