All-purpose infantry:
Warrior 1.1.1 -> Spearman 2.2.1 -> Pikeman 3.3.1 -> Fusilier 5.5.1 -> Infantry 7.10.1 -> Mechanized Infantry 14.14.2
(Nothing) -> Mobile Infantry 8.8.2 -> Mech Inf
These units are used both for attacking and for holding ground until defensive infantry units can come in and defend the place. Mobile infantry is intended to support Infantry in the world war era, using its speed and attack to push the front while Infantry and Machinegunners hold it.
Defensive infantry:
Archer 1.2.1 (2.0.1) -> Crossbowman 1.3.1 (4.0.1) -> Musketman 2.4.1 (6.0.1) -> Rifleman 4.6.1 (8.0.1) -> Machinegunner 4.10.1 (10.0.1) -> Modern Infantry 10.16.2 + Stealth Attack + Pardrop range 3
Mostly used to defend cities and other positions. Almost all units have defensive bombard. Some can also be used for limited offense, like the Rifleman and the Modern Infantry. Modern infantry is both intended to hold strategic positions and to harm and destroy specific targets globally. They have 2 movement points because they rarely ever march - they usually use rapid air and land transports to get from place to place. This is also why they have paradrop to a short range. They can be paradropped out of an outpost to intercept an incoming enemy force before it can do lasting damage.
Guerilla:
Swordsman 2.1.1 -> Partisan 4.4.1 -> Guerilla 5.8.1 (8.1.2) + All as roads
(Nothing) -> TOW Infantry 6.12.1 (12.0.1)
Cheap units used to put a dent in the enemies' defenses for main force of all-purpose infantry to take out. Not very capable of holding ground, but very capable of annoying the enemy and capturing workers. The modern Guerilla has low attack but a solid offensive bombard that can be used to harass nearby units.
Shock troops:
Chariot 2.1.2 -> Horseman 3.1.2 -> Horse Archer 3.2.2 (2.0.1) -> Knight 4.3.2 -> Cavalry 6.3.3 + Blitz
(Nothing) -> Tank 16.8.2 + Blitz -> MBT 20.14.3 + Blitz
Expensive, fast, and powerful units used to knock out the toughest defenders so that the main force can do its job.
Marines:
(Nothing) -> (Berserk 5.2.1 ->) Marine Infantry 8.4.1 -> Marine 12.6.1 | (All have amphib obviously.)
Marine infantry is the WWII marine of the sort that stormed the beaches of Normandy. Basically just infantry trained in amphbious assault. It should be available with some WWII tech and upgrade soon after to the modern Marine which is better trained, better equipped, and more capable of holding ground for long enough that better defenders can come in and make sure the Marines' work wasn't in vain.
The setup probably requires some tweaking, but I feel I combined both setups pretty well. It makes alot of sense in this setup that I suggested.
EDIT: It's important to see which units are contemporary with which units. So here are the groups that should end up interacting. (Also added TOW Infantry. =X)
Warrior
Spearman, Chariot, Archer
Spearman, Swordsman, Horseman, Archer
Spearman, Swordsman, Horse Archer, Archer
Pikeman, Knight, Crossbowman, Berserk
Pikeman, Musketman, Knight
Fusilier, Cavalry, Musketman, Partisan
Fusilier, Cavalry, Partisan, Rifleman
Infantry, Machinegunner, Mobile Infantry (WWI)
Infantry, Machinegunner, Mobile Infantry, Tank, Marine Infantry, Guerilla (WWII and the beginning of true combined arms)
Mechanized Infantry, Modern Infantry, MBT, Marine, Guerilla, TOW Infantry, (Modern times and the ultimate realization of combined arms)
This helps clarify how the units will interact. I probably misplaced some of the things (like the Partisan, the proper location of which I didn't really remember.)