However, being able to just produce them individually causes a few issues for each unit, mainly:
Skirmishers: A bit too powerful, and discourage production of master skirmisher tech.
Horseman: (I don't think knihts existed in Roman times

Swordsman: Basically same problem as horseman
Archers: They were designed to be literally identical to saggitarus, so they really don't serve any purpose on their own besides looks lol.

So, I think auxiliaries really need to be created randomly from a single thing, or else we need to kind of rebalance all of the units (mostly nerfs, besides the archer).