Personally, for units I use rusted pack file manager instead of assembly kit (you need to google it from github). The tables which needs work from data_db in data folder game are: units_custom_battle_permissions_tables (for custom battles as name suggests), units_to_groupins_military_permissions_tables (for campaing) and building_units_allowed_tables (alternative to previous one, able to block recruitment from building). To better understand whole process I suggest to analyse in RPFM data of existing mod, for example brilliant COHORTROY Unit Pack ;)

Thank you this helps, I find poking around the files with criss-crossing references etc. hard to figure out which to use etc :)