An idea hit me, and I thought I'd toss it out for you to consider...
A nifty little reason to toss in as to *why* the Spec-ops types are on a semiseperate rank structure. After the Commonwealth war, it became possible to create them in some numbers. (This was especially true after the techniques had been refined to the point that it was a bit easier and more reliable to do so. Seriously, people, wait a few years on those bone grafts. It doesn't cost you any more, and it cuts way down on that particular washout rate.) Anyway, some bright boy up in the head shed figured to give the Spartans a rank commensurate with their responsibilities. They went to the Master Chief, figuring to promote him first, and promote the rest around him, and he turned them down. He liked the rank he was at, and had no desire to promote further, or to take the very real risk of getting stuck behind a desk. They went to the rest of the surviving Spartans, but they refused, to a man (and woman), to be promoted past CPO. The Master Chief was the best of them, and they got the idea that it wouldn't be right. When Spartans get ideas, it can be rather hard to sway them. The Spartan "command structure", such as it is, maxxes out at E-8... and there is only one Master Chief.
(Incidentally, if you decide to run with this, it would mean that a CPO would usually be the ranking member of a cell, with a rank-equivalent of more like CW4 or CW5, and might well report directly to a naval Captain. My previous estimate assumed that ships with a significant Spartan element would generally have a Master Chief as ranking member)
(if any of this is too deep in the weeds as far as ranks go, feel free to ask, and I'll happily explain.)
A nifty little reason to toss in as to *why* the Spec-ops types are on a semiseperate rank structure. After the Commonwealth war, it became possible to create them in some numbers. (This was especially true after the techniques had been refined to the point that it was a bit easier and more reliable to do so. Seriously, people, wait a few years on those bone grafts. It doesn't cost you any more, and it cuts way down on that particular washout rate.) Anyway, some bright boy up in the head shed figured to give the Spartans a rank commensurate with their responsibilities. They went to the Master Chief, figuring to promote him first, and promote the rest around him, and he turned them down. He liked the rank he was at, and had no desire to promote further, or to take the very real risk of getting stuck behind a desk. They went to the rest of the surviving Spartans, but they refused, to a man (and woman), to be promoted past CPO. The Master Chief was the best of them, and they got the idea that it wouldn't be right. When Spartans get ideas, it can be rather hard to sway them. The Spartan "command structure", such as it is, maxxes out at E-8... and there is only one Master Chief.
(Incidentally, if you decide to run with this, it would mean that a CPO would usually be the ranking member of a cell, with a rank-equivalent of more like CW4 or CW5, and might well report directly to a naval Captain. My previous estimate assumed that ships with a significant Spartan element would generally have a Master Chief as ranking member)
(if any of this is too deep in the weeds as far as ranks go, feel free to ask, and I'll happily explain.)