I too have a WH40K flavouring although the full imperium list I use is filled out with Manticore, Nelsons navy and so on, its been in use for every 4x I've played over the years.
If your adding a 40k flavour I will attach an an excel sheet (saved as csv so you'll need to adjust the columns) with the design names I import every iteration, which aren't all canon, but do have the flavour (has Imperium, Traitor, Eldar, Dark Eldar, Necron, Tyranid, Tau and Ork). I say "design names" as I make a design and call the first ship of that class to be built that name. Maybe you can glean some stuff from it to flesh out the theme.
The ranks are taken from the Lexicanum so are pretty acurate.
Ground troops get named depending on type as well, Marines become Legion I, Legion II etc and are generally three Battalions consisting three Marine units (1st Company, 2nd Company etc) and an assault unit named a "Tactical Dreadnaught Support", the fourth battalion are heavy Heavy Assault Battalions named Heavy Support Squadrons, Ive included that on the csv too. I did go through a phase of naming the Heavy Assault Battalions as Titans, and assault Battalions as Scout Titans but that can turn out to be a
lot of Titans in a long game. Garrison troops become Arbitrators under a Brigade Headquarters renamed Adeptus Arbites Provost. Imperial guard are left as is, usually just prefixing their name with the Division (Like 1st Terran, 7th Armd etc).
EDIT: Updated the Imperium.csv as I'd copied over the design/shipnames from the wrong place