Looks good so far! As far as the setting goes, I can't spot any problems. Any given world in te Imperium is bound to be drastically different to any other, so you have pretty much free reign.
The ten men of Squad Ottowa were already clothed in the navy blue/white bodygloves of the Sousgrenier Arbites
Minor issue, but I don't know if blue/white is correct. Maybe stick an 'and' in.
Lieutenant Ranuel, up in the cockpit, visibly started as the Valkyrie next to his exploded.
'visibly stared' seems a little... well, the 'visibly' is unnecessary. It just doesn't strike me as the right word. Because, any staring is going to be visible, if you get what I mean... um... hmm.
I would say 'stared gaping' or something like that, describing how the character stared. 'visibly' doesn't really do that.
The shock of this is somewhat lost, and I don't know if you're going for that effect. If the Valkyrie was close to theirs, maybe you should add a little more reaction from the crew and squad. Maybe their Valkyrie is going to be hit by debris, or rocked by the explosion.
It had taken a missile directly to its crew compartment, and had been utterly gutted, cockpit to drop-ramp.
This is probably just personal preference, but I would say 'been gutted ccokpit to drop-ramp'. But that's just me
The tall, gangly Saall pitched forward and smacked his head on Brej’s knee, who was sitting opposite him. He got up, grumbling sourly, and armed his autorifle.
The action here is good, but it doesn't feel as if the Valkyrie is spinning over and over. The fact that Saall's grumbles are mentioned... sort o influences the reader into hearing them, if you see what I mean. So all the noise associated with large vehicles rolling is lost, and all the reader hears is grumbling. Plus, the fact that he arms his gun seems to imply he's not that interested in clinging to the ship for dear life.
Maybe the Valkyrie has stopped rolling at this point, but it may be good to mention that. If not, I would suggest something like 'He forced himself back up, adding sour grumbles to the chaotic noise of the passenger compartment'.
I would also question the necessity of putting the Valkyrie's signifier here. It breaks up the action a bit. Maybe move it to a less intense part of the story.
Ranuel banked hard to the right. At his side, Hommus was firing the transport’s lascannon, cracking off shots at the rapidly closing harvester. One round hit a long-barrelled autocannon the rebels had set up on the roof of the hump-backed harvester. Hommus fired again, at a gun near the massive threshing blades at the front of the goliath, but missed.
Again, nothing too wrong, but, IMO, the general chaotic nature of an air battle is lost when the reader get's crystal-clear descriptions of what happens. It's cool to mention whether the gunner's hitting or not, and what he hit - it helps the reader visualise the scene immensely. But they're not visualising the gunner desperately drawing a bead on anything that looks gun-shaped, seeing the crosshairs jitter and shake as he tries to line them up - they're visualising a lascannon shot destroying an autocannon.
I would change the focus to the gunner, rather than the targets.