In vanilla rtrpe the first cohort is recruited after the marian reform from the last level of the barracks(Roman Citizenship) which can be built with no restrictions anywhere on the map. If you want to make them recruitable from the previous level you need to open the file export_descr_buildings from Rome - Total War/data. Or from Rome - Total War/RTRPE/data if you have the mod switch version of rtrpe. Make a backup of the file first. Then find this building:
Code:
muster_field requires factions { romans_julii, slave, }
and make the first cohort recruitable from there as it is from the next level. I.e. you add this line:
Code:
recruit "roman legionary first cohort i" 0 requires factions { roman, } and marian_reforms
under the one of the regular legionaries. Which is this one:
Code:
recruit "roman legionary cohort i" 0 requires factions { roman, } and marian_reforms
Save and exit. Make backups first(of your saved games too)so in case something goes wrong you don`t lose anything.