A thought regarding Seven: we've seen that there are three ways to get into Starfleet.
1) Go to Starfleet Academy. This is the path most officers take, though the problem is that the Academy only accepts the best and the brightest (as we saw with Wesley; most candidates have to apply multiple times before they successfully get in), and non-Federation citizens (like Nog) have to be sponsored.
2) Enlist. This will get you into the fleet, but you will not be a commissioned officer. This is the path Chief O'Brien took (he was Chief Petty Officer on DS9, and noted that a fresh-out-of-the-Academy ensign outranked him).
3) Join an equivalent organization recognized by Starfleet, rise through the ranks, and transfer over. We saw this in DS9 when Kira went from a Bajoran Colonel to a Starfleet Commander, and in Lower Decks, when T'Lyn transferred from the Vulcan Expeditionary Fleet to Starfleet. Presumably, Janeway forged an ad-hoc equivalency agreement with the Maquis off-camera when Voyager was stranded, allowing Chakotay's crew to transfer over with their ranks intact (probably facilitated by Chakotay already running his crew to a formal standard).
For Seven, it's possible the Fenris Rangers were such an organization, in which case her previous status as an agent would have a bearing on her Starfleet rank.