So i was thinking if i should make a new thread for this and i probably will. still i think your mod would be the first mod that could benefit from this idea.
i would strongly argue that the mode of research as it is handles in Civ 4 is not aplicable to "Era" spanning mods. like yours.
A limited era project, similar to the "Greek World" mod that shipped with the game really really does not do well in terms of technology. Sure you can see something like "Nuclear Fission" somewhere in the tech tree, yet it will take a million turns to research and you only have soo much of land to conquer
The "American Revolution" mod is even worse.
You can not even access the research screen in that scenario.
It is limited to 100 turns total, and there was no drastic change in technology there.
However taking out research completely makes the replayaility of the mod virtually null
So here is my proposal
A number of technologies and appropriate buildings would need to be disigned to simulate era appropriate buildings that add incremental functionality to the cities. Similar to what medieval total war and Rome total war do with their research/building cues.
in there both are combined. which in not required for the concept to work
Technologies / Buildings
-Spearmaking (tech) Allows to build the building ("Spearmaker ) which allows to train spear troops.
-Horse domestication - allows Stables allows to train better horses suitable for warfare.
if both buildings are built you can build either spear cavalry or chariots. Chariots might require Smihy
-Axe Smith (Allows to build Axemen)
Also building upgrades would allow to use promitions creatively as they coudl be tied to a building
Build "College of Medicine" and all troops could either get the healing promotion or you could build specialized medical units. although this is for late eras. this promotion would be free and not require experience to be spent
Muster field woudl allow to train basic troops and would be a prerequisite for barracks which allow to build better troops and would be a prerequisiste for royal barracks.