It's in the game's data folder and it's called export_descr_buildings.txt
a) BACK THE FILE UP.
b) There is a building tree called "barracks", which has various levels (one for muster field, one for militia barracks, etc). Every level has recruitment lines in it, which define what kinds of units are recruitable from it. Now, in the "army_barracks" level of the "barracks" tree, there is a line like this:
Code:
recruit "roman triarii" 0 requires factions { roman, } and not marian_reforms
This does not exist in the previous levels of the barracks building, so the unit (triarii) is only available from the army barracks onwards.
Therefore, you need to copy this line to the previous levels recruitment sections, in order to make the unit recruitable from them.