I can appreciate the nitpicking - sometimes a pretty small detail can be irritating (as an example, I always found the confusion of terms in Tolkien's orc/hob-goblin/goblin distracting). I also won't pretend to be an expert in military terminology... my only real experience was in the Combined Cadet Force as a kid, where I was overly amused to spend some time as first a (Cadet) Able Seaman and then a Leading Seaman (you may have noticed that my sense of humour can be a little immature sometimes!

) I'm glad you brought it up, though, cos I spent a lot of time thinking about it!
The SpaceCorps military hierarchy is based on - but very different to - ours, reflecting a 1000 years of development and the imposition of effective military rule. It is not supposed to reflect the same chain of command as our current structures - it's a fictionalised idea of what
might be. As a reference point, the time gap is the same as that between us and 1019, when roles were extremely fluid, but an 'Captain' often served in the role of a general, a 'Major' could be anything from a butler to a field marshal, and a 'General' was normally a sovereign/semi-sovereign ruler like a Duke (Dux), Earl (Jarl), or Amir.
As a side note, the creation of a multi-species Union has fundamentally overturned the sort of class structures that led to our current two-track approach of enlisted-officer, and there is no longer the sort of patronising approach that holds that the 'lower orders' are only capable of tactical rather than strategic thinking.
SPACECORPS:
All traditional branches of the armed forces (e.g. Army, Navy, Airforce, Marines, Intelligence) have been merged into a single organisational structure - although residues persist in the 7 branches (e.g. Forward Infantry is the descendant of the Army, FliteCorps of the Navy & Airforce). The organisational difficulty and power struggles which were involved in merging every armed force in the world into a single hierarchy have led to a general reordering.
One effect of the stratocracy has been to eliminate the currently understood distinction between enlisted and officers. Everyone (even Lesser Species and conscripted) begins as an enlisted Cadet, sending the message that every recruit has the potential to become an officer - and thus part of the ruling caste within the Interstellar Union. (
"Service guarantees Citizenship" 
)
Once a Cadet graduates into one of the Branches, they enter a weighted hierarchy (the same way NATO currently balances ranks, but with more emphasis on relative competence and authority). Thus, a Forward Infantry Corporal is equivalent to a SecuriCorps Constable and a FliteCorps Ensign - with only the seniority of the Branches making a difference.
Where it comes to Sergeants - yes! That's what I'm trying to reflect!

We all know that most Sergeants are worth a dozen Ensigns and Second Lieutenants - because they actually know what they're doing - whereas Ensigns are still finding their feet as leaders and necessarily rely on their Sergeants to get anything done. In a Nietzschean structure, that has been necessarily reflected in the power wielded by those individuals, and Sergeants now outrank Ensigns. Another example is over in MediCorps - in which a Cadet progresses to Medic, Resident, Nurse, Doctor, etc, and in which a Nurse outranks a Resident.
Essentially, SpaceCorps demands that people start at the bottom and work their way up through the ranks by demonstrating their competency at each stage - with no fast-tracks, graduate schemes, class-structures, qualification-gatekeeping or nepotism built in. That's partly a riff on the themes of Starship Troopers, partly because it fits nicely into a game structure of progression, and mainly as a fun experiment in world-building for me.

lol - that was crazy TL;DR, but I'd be really interested in anyone's thought on the points raised (if they managed to wade through the entire thing!) Now's a good time to raise any points, since we're soon about to start heading into the stat-building part and the ranks/branches will be much more relevant to gameplay!