I worked in a costume shop
The shop I worked in is a local company that does rentals and sales for both theater and fun & holidays as well as manufacturing mascot type costumes for schools and businesses. They are at least twenty years old. They grew and added on over the years, moved, bought outside storage sheds, etc. They move & shuffle things according to trends from year to year. This is more or less how things were organized when I was there. All of it was terribly crowded, which is the normal state for most costume closets/ rooms/ buildings. Last count they had over 40,000 costumes.
Within each section hats were on shelves above the clothes they went with. Shoes, belts & jewelry were on a corresponding wall.
Mens & womens 'every day' clothing chronologically by era.
Eras where there were lots of costumes representing different class levels (Renaissance, medieval) we put peasants first & then ranked up within the gender group to fanciest/ royalty. The vast majority of this was used primarily by theater groups, with the exception of Renn fair goers who wanted something special to wear.
Main room, second section:
Military costumes from around the world, also in order of era. One of my favorite sections as most were real.
Main room, third section:
Uniforms & professional clothing, all eras, by profession. (so our antique fireman jackets hung next to the new modern firemen jackets, then police uniforms of all types, etc)
Main room, last section: Ethnic costumes (mostly stuff used for dance & ceremony- lots of rentals to schools doing cultural events & study) sorted by nation/ ethnic group.
Section 1: all fur suit costumes: animals, TV characters, whatever.
Section 2: Fantasy costumes: both classic fantasy stuff like Halloween witches (lots of trashy & sexy Elvira type dresses) as well as specific not-a-real-person costumes (Mother Goose, Pan, Cinderella, Star Wars characters, yadda yadda)
Section 3: Childrens costumes: Mostly period & fantasy as there isn't as much call for childrens rentals.
Dance Room: 20th century fashions & all chorus girl type stuff (which tends to be small sparkly leotards which fit nicely in boxes instead of on hangars)
What lived out in the sheds: materials for making costumes (foam, fabrics, etc), very large bulky things like the plant for Little Shop of Horrors
, currently out of trend fur suit kiddie characters, recently past popular formals & other regular clothing purchases to be used in the future.
Hope that helps! It sounds like more than anything you need more space for it all.