In another forum we discused about a system where ships need an captain.
Tiny hull ships needs an Lieutenant (junior grade) or Lieutenant
Enormus hull ships needs an Captain.
Also a fleet needs an Commander.
Tiny fleets (e.g. Logistics 15) needs an Commander
Large fleets (e.g. Logistics up to 55) needs an Rear Admiral (lower half)
Super large fleets (e.g. Logistics higher 55) needs an Rear Admiral or Vice Admiral
Proberly there should be an fleet command to play with large numbers of fleets - for the fleet command you need an Admiral or Fleet Admiral
Military Academies trains Lieutenants. Ship Battles or special missions level up the Lieutenants to Lieutenant Commanders and so on. To get an Admiral - or Fleet Admiral - you need to level up your captains - and take care that your ship commanders aren't killed during fightings or you will never have an Admiral 
Later techs will bring up special forces Academies which allow directly to train low level fleet commanders.
For ranks take a look here: https://en.wikipedia.org/wiki/Ranks_and_insignia_of_NATO_navies%27_officers