Luckless

Actually an absolute monarchy feudal system makes a lot of sense for a vast civilization. (There are very much major problems with democracy, namely it is hard enough to get twenty people in a club of shared interests to fully agree on something, let alone a few hundred billion spanning dozens of planets.)

At that scale the emperor doesn't need to make direct decisions, he handles high level elements and delegates the actual work and details to others. The emperor is the one setting goals, others do the work to actually achieve them. The key point of the emperor is then to basically bully everyone into making sure the job gets done, and ensuring the right people are in the right places at the highest level. He becomes who the lesser people answer to when things go wrong.

Factories in one sector are under preforming, and that is impacting Imperial Economics? Emperor yells at his minster/lord/whatever in charge of that region, who in turn yells at his subordinates, who yell at their subordinates, etc, etc, and eventually the problem is fixed, or another round of yelling takes place and a few heads roll. If it continues with changes (that the emperor doesn't even really need to know about) at the lower level not having an effect, then the emperor 'steps in' and puts someone else in charge of the whole mess.

The question is how large are the units you are considering dealing with, and how much structure is there in them?

Ultra simple setup in a very primitive rebuilt society would likely only use three to four ranks at best, if not just "Leader and Followers", simply because the higher ranking titles are born out of a need for organization.

Something as simple as:

Gunman/Rifleman/Soldier/Private/Warrior/Guard/etc

Sergent/Sarg

Captain

Commander

Throw in special titles such as "Runner" for say a sergent who was trusted with carrying orders between units for added flavour.

Personally I think it is just weird when writers try to suggest too complex of a force with too many ranks being used, and then have no actual need for that kind of structure. 

I can think of a few dozen empires in history that were hereditary, but I can't think of one that wasn't besides the Holy Roman Empire, and even they were essentially hereditary for the most part.

Volumes of space vs volumes of space. Many smaller ships become grouped together in an 'operation space' while your largest classes of ship will take up an entire space to themselves. Offer bonuses like combined arms for different weapons, systems, and support functions operating in a single volume, which means your larger ships are more likely to be able to carry all the gear to get a given bonus all on their own, but the smaller ships become at risk of losing their bonus if a given ship gets taken out, while the larger ship will take damage and degrade, but is less likely to lose the system and completely lose their bonus.

The thing is, nothing says they can't really be both. The universe laid out by Frank Herbert, and continued by his son, have this beautiful mix and balances of power. The Noble Houses are very much driven by economics and the management of whole planets, while the corporation side focuses more on larger scale trade and the economics across the whole empire. Your nobile houses are more like major family businesses that keep an iron fist on local markets, and the few major corporations are almost their own secret societies driven by profits. 

So the emperor's job becomes one of pitting other players in politics against each other, and it is a very delicate balance. Offer too much support to one side, and they will rise over you and cut your throne out from under you. Be too ineffective, and the nobles begin to work together and over throw you.