If you've an alternative for what Units can be called that fits better, it's not too hard to change ti at this point. In text anyways.
Company - widely used in the Middle Ages, although more commonly used to describe mercenary companies like the famous White Company.
Warband - As Cipheron pointed out, this could well be used to describe the smallish groups of soldiers of the early Middle Ages. If I remember correctly (and I'm pretty sure I do), the game is actually meant to represent an early Middle Ages society where nobles commanded small groups of soldiers.
Band - a variation on Warband.
Personal Guard - roughly describes the purpose of the typically fewer than 100 soldiers that each First Ones tends to lead most of the time.
Companions - a variation on Company.
Men-at-Arms - a reasonable description of a group of Middle Ages soldiers.
Yeomanry - same as above.
From that selection, I'd probably go with either Company or Warband. We know that people in the Middle Ages legitimately used Company to describe their groups of soldiers. Alternatively, Warband does describe pretty well the smallish numbers of soldiers that most First Ones command most of the time.