In my mind supply should be based upon : Material used ie steel/Aluminum/oil/Uranium for Battleship for example then supply by population so to crew a battleship would be less than crew for an infantry unit and more than a fighter
Battleship uses 1 or more oil / 1or more Aluminum a low% based population supply (2000 crew average for battleship)
A fighter 1oilo r more/ 1Aluminium , a single pilot extremely low based pop supply ( 1 or 2 pilot(s) /1 rear)
A infantry unit a medium or high% based population supply (5000 for average brigade up to 10,000 for some )
Supply bonuses could be given in circumstances such as Policy Paths, Warlike civs, happiness of populations, type of Ideologies, going tall vs wide etc
Industry can have more "Ships" , "Planes" - less popsupply
Imperialism can have more "Melee units" - "drafts"/conscripted/volunteers more %pop
Supply negative bonuses.. Unhappiness, going rationalism populations prefer science path rather than defence.
also if loosing many melee units could have an impact on population for every 10 units loose say -1 pop to all cities loosing too many ships/planes affect happiness and increase %gold cost
just throwing some ideas