Don't forget, in considering Build Times, costs, maintenance resources, that the relative size of military units has to also change dramatically if the game is to stay playable.
The largest regular units in Ancient Era were about 600 men (or 120 chariots in Egypt), which is slightly smaller than the average Battalion in the 20th century (Modern Era). The standard 'Infantry unit' of the Modern Era was the Infantry Division of 12,000 - 18,000 men, a 'unit' 20 - 30 times larger than that of the Ancient Era.
So, I think you have to consider all the Build Times, Resources, Population, etc required to form military Units are also on a 'sliding scale' throughout the game, if you don't want your modern Super State to have a military force smaller than that of 1940 Latvia.
As to population requirements for units, I think we have to divide the basic population into two categories: the basic Population Points and Specialists. Consider that each population point represents both the bulk of the population (throughout most of history and game time) working the land, and a much smaller group of people with special skills, education, training, manning the structures in the cities - the Markets, Temples, Palace administrators, Harbors, etc. For most of history the military was a tiny percentage of the total population (except, notably, among the pastoral nomads, where just about every adult male was potentially a 'warrior', having riding and shooting skills as part of his 'civilian' herd-guarding job) so that building a military unit would not affect the overall population 'points' appreciably. For game purposes, though, it could be modeled to require Specialist points - to affect how well your City works even though the bulk of the rural population is untouched by it all.
This, of course, changes when a city and its region is sacked or razed and the entire population dies, disperses, or is hauled off to the slave markets. It also changes at about the Industrial Era when mass conscription become normal and, again, virtually every adult male is potentially recruitable as a basic Soldier. This ties in neatly with the sliding scale of units: Ancient/Classical/Medieval/Renaissance armies typically were 10,000 - 40,000 men, rarely over 100,000 because it was simply too hard to keep 100,000 men and their animals fed with the technology available. Napoleon in 1800 - 1815 regularly fought battles with over 100,000 men on both sides and Imperial France maintained total forces in excess of 500,000 men for most of that period. By a century later (Modern Era - 1914) France went to war with over 3,000,000 men mobilized, and that was such a large percentage of the male work force it substantially affected industrial and agricultural production.
So, up to the Industrial Era recruiting a military should not affect the overall Population - unless you engage in continuous, long term warfare, which could be modeled by reducing your population increase rate substantially as you remove young males from the population continuously, as happened in Sweden in the 17th century (Renaissance Era). For balance, this could be linked to adopting some form of conscription (as Sweden had), which gives you cheap military units but also is balanced by slowly reducing your population to a static number that hurts you in the long run (as it did Sweden in the 18th century).
From the Industrial Era (Universal Conscription) on, recruiting the modern military units that average 10,000 or more men per unit bites into your working population Immediately, and part of your 'war planning' must include how to balance the work force (both Specialists and rural Population) for production, agriculture and all other civilian requirements with the population going into uniform. This became a nearly overwhelming problem for almost all the participants in WWII: Germany had an industrial manpower shortage that materially reduced production from 1941 on, the Soviet Union ran its factories after 1941 almost entirely with women and children as industrial workers (and agricultural, and recruited almost a million women into uniform), while Great Britain was disbanding whole divisions in 1944 for lack of manpower to maintain them - and a large fleet, and a large strategic bombing force, and her industry, and the USA started integrating black troops into the segregated regular army divisions because, by late 1944, they were the only troops left in the USA trained as infantrymen: everybody's normal 'manpower pool' was reaching the bottom by the last year of the war if not sooner.
