1. Imperial Uniforms will vary by period. This has been quite likely in every military force in history we know of so far, so I see no reason for it to differ in the various ages of the Imperium(s).
2. Imperial Uniforms will vary by location. There may be a standard, but the military brass and military units being what they are, there will be exceptions granted left, right and center. There will be preservation of a number of traditional uniforms simply because allowing them to persist, with attendant espirit de corps and unit history, is part of building elite units. No Emperor in his right mind is going to try to turn every fighting unit into the same thing. Even Rome had divergences and it was more consistent than most. Look at the British Empire - uniforms varied all over the place (as it did with most Colonial and far-flung empires). And if one Emperor changes this, another may well change it back.
3. Regardless of what regs say, uniforms will vary in use quite a bit. I know what Canadian combat uniforms are meant to look like, but I also know that when they sent our boys to Afghanistan first, they only had the green CADPAT. Soldiers, being pragmatic, splotched them with paint in desert tones to get closer matches to local terrain. (That's combat uniform, but still....)
4. Uniforms would have to vary dramatically to account for anatomy of alien races and their visual and other perceptors, esepcially in areas where the Imperial officers and NCOs may be the only humans (if there are that man) while most of the troops are ... something else.
5. What does a dress uniform look like for a Dolphin, a Githiasko, a Virushi, etc? Do they even have a concept of dress uniforms? Or uniforms at all?
Yes, some of these are tangential to what you are trying to do. And the Imperium is somewhat humano-centric, despite having a ridiculous number of different intelligent species serving in one capacity or another.
6. Navy Work Dress/Shipboard Uniforms: Don't like the depictions with zippers and seperate jacket and pants.
Seperate jacket and pants - no point in this. The shipsuit coverall makes more sense. If you must have the jacket and pants look, then the pants must have a moderately broad area of touch-seal around the top (can be hidden under the jacket) - think like the top of "Hammer pants" - and the jacket must come down to cover this and the interior must have the quick-seal mating material. So effectively you turn the jacket and pants INTO a coverall. The only reason to have them seperate is you can remove the jacket at times.
Zippers in a tailored vacc suit? Not likely. Some sort of touch-seal closure. And I would think that the point-down style of collar would just not happen. You'd have a priests collar that went around the neck (perhaps you have to pull a piece across to close the loop) and that the soft helmet or hard helmet could mate to. Similarly, wrists would be tight so you can mate to the gloves.
If you are assuming they are carrying the soft helmet, the collar/back of collar should be thick and/or bulgy and that's probably where the helmet is stored, to deploy like a Mustang Floater vest (opening out).
The gloves are probably stored in one cargo pocket on the leg (or two if you prefer symetry). If you get a breech to fast vaccuum (large hole blowing out most of the air immediately), you've got about 20 seconds to get sealed or you'll pass out. The uniform probably has sensors in it to pop the collar holding the hood so you can just reach back, pull it forward, run your finger around the neck joint, and its on. Then grab your gloves from the pocket.
Also your O-7 to 0-10 ranks probably should have the silver/gold (or whatever) colours on the stars reversed and maybe add a 1 pixel outline so you can see the edges versus the yellow (or gold if you will) epaulet. I'm assuming these are going to be precious metals as the admirals are likely to be just that sort....
Keep up the good work (my comments are all meant in the spirit of constructive criticism).
