He just had some old pacifistas with him back at archipelago.
–- Update From New Post Merge ---
And there's no reason to not give a leader a proper rank.
I said I don't think he has a conventional rank, like your bog standard captain, commodore etc.
I can't imagine Sentomaru being particularly willing to do the menial jobs of a typical navy officer.
He's most likely still Vegapunk's muscle, and working with the Marines Pacifistas but now in an official capacity endorsed by the marines.