Legions are made of 5 regiments of 1100, split in to 4 company's of 270, each with 4 platoons of 50, made of 5 squads of 10 and the left over 20 are support.
This is a plenty viable force distribution. The squads are extremely large, which slightly reduces tactical versatility but slightly increases it in other ways. Attaching support to the company level instead of the squad level is an interesting choice.
A camouflaged Jumpsuit with cross bandoleers, black Jump boots and studded bracers with studded fingerless gloves.
Higher ranks get bigger taller boots leading to bucket top boots for regiment officers and thigh high stilt platform high heels for generals.
You can definitely delineate rank by boot-size, although if the Inquisition catches on that the top-brass are all wearing thigh high stilt platform heels they're likely to assume heresy is involved. Of course, they're already going to assume you're heretics due to rebelling against the Imperium of Man but that would solidify it.
as for uniform, I would like something simple but distinctive, but what about the idea of if a guardsman does good, they are allowed to customize their uniform? just spitballing
Troops should be well rewarded with medals, skull badges and freedom from regulations for there rewards.
Freedom from regulations and uniform customisability as a reward for competence are both very possible.
What about instead of taller boots we give the officers taller hats, with every rank up their hat grows taller, then when we have a massive army our highest ranking officers will have hats that can be measured in kilometers or miles.
You can also definitely delineate rank by hat-size and this is more popular with the Imperium, or at least, the Ecclessiarchy, but that alongside the Vox Meritum's secondary objective of doing away with the Imperial Cult may alarm them further. The human neck can only support so much weight, however, and sufficiently massive hats will require cybernetics or genemodding to prop up, as well as being closer to tent-weaving than tailouring.
I'm concerned about making our command structure to easy to deduce, as that is asking for our officers to get targeted. For parade uniform though, I'm absolutely on board for dapper hats and boots.
Soldiers should all know who their direct superiors are anyway, so they won't need fancy signifiers to remind them.
Extremely distinct uniforms are likelier to be targeted by enemy fire but if they're separated by larger boots and hats they're also likelier to be away from the frontline. At a certain point, it's slightly impractical but there are many regiments of the Imperial Guard that get by with slightly impractical uniforms. No matter what you go with, the fighting legionnaires (name-pending) massed on the field will be capable of fighting.
The issue with that is in a high casualty environment, a average soldier, trooper, whatever we decide to call our grunts may end up with all of his direct chain of command as casualties. In a situation like this having identifying markings for officers and NCO’s will make it easier for the lower enlisted to identify who they should be reporting to in the event they find themselves separated from there assigned leaders either via casualties or fog of war.
This also has me thinking what is the status of our communications equipment does every soldier have some form of commo or is it restricted to RTO’s and leadership ranks. If the latter is the case we should seek to correct it as soon as possible
The Vox Meritum's forces do have access to comms but like the Imperial Guard, long-distance communications are limited to the large, cumbersome but resilient vox casters held by specialists. If you wanted to add smaller, short-distance communications like a walkie-talkie, that'd be possible and make men less likely to be disoriented in certain types of terrain, but also likelier to have their comms intercepted against technologically-savvy foes. Light Comms would require an additional 1 Resources per regiment with no additional upkeep but Heavy Comms, adding a man-portable vox caster to each squad, would require 10 Resources per regiment and increase upkeep by 1. Due to your force's guerilla roots, they already have a fluid coded language not present elsewhere in the Imperium though it could likely be "decoded" through torture fairly quickly.
As for a flag, what about a hand/multiple hands reaching for a star? The hand is the people/citzens, and the star represents the concept of the merits or goal or whatever clearly too tired to expand on this. but the flag should give the sense of reaching for the stars being totally possible in our society.
EDIT: add a cybernetic hand to the flag, gotta show appreciation for those cog bois for supporting us
I'm going to give you all a couple of more days to decide on the details of everything, but so far the hands reaching for the star flag is in the lead, as it's the only suggestion. Likewise for the semi-customisable jumpsuit+bandoleers, with increasingly large boots and hats for rank. What colour are the flag and your uniforms? Your troops may be wearing camo but there are many regiments that don't, and don't have any distinction between parade uniforms and active uniforms.