If there is an early leader, then Swordsmen or Medieval Infantry will be put into the army.
But what if you get a Leader even earlier than that?
My current solo-game (Large, 60% Archi — I think — Emp, as Japan) began as a failed AW-attempt*.
Although my spawn-point was in quite a nice section of the largest continent (lots of Grass and Hills, albeit only two 1-tile lakes, separated by 1 tile, for freshwater), I got visited very early on by all 3 of my neighbours (America, Babylon, and Holland), and thus was 'required' — per AW-rules — to DoW them (triggering Babyon's GA when a Bowman won a fight).
During these initial hostilities, I got an MGL from an eArcher (which itself died over the following interturn). Since all my towns were building units, and I also didn't yet have enough towns for my FP, I decided to build, fill and fight an Army, so I could unlock (and build) HE, and thus increase the probability of generating further MGLs. However, although I had WarCode as a starting-tech, and had either researched or traded for Bronze already (I allowed myself 1 opportunity to trade before DOW-ing), I did not have IW (IIRC, I was researching it, but was not far into it) — so the only units I had available were Archers or Spears (or Warriors!).
Since I have seen the AI attack a healthy Zerk-Army on a Hill, using only LBMs and ACavs, I was not convinced that a D=1 Archer-Army would not be attacked (and killed) by Archers/Swords. So, following the 'single-unit-type doctrine', I went with 3 Spears*.
Obviously that meant that my first Army was really only good for defence, and I could only force the AI to attack it (and grant me an Army-victory), by garrisoning it in my frontline town (which was on a Hill, Walled and Barracked). At the time I figured that would be OK, though — because later on I would be able to use it as an invulnerable stack-anchor for my advance into enemy territory (or at least, for moving my frontline further west, to hold a Hilly semi-chokepoint until I could Settle there myself).
But I'm now wondering if I would have been better off putting a mix of units into that first Army: say, 2 Archers, 1 Spear — or even 1 Archer, 2 Spears?
Or
possibly even keeping the Army empty until I had Swords (this would have required 10-15 more turns of research, though).