Im Retrosaurus, about Cultural change, iceni doesn't have cultural modifiers or economy bonuses/ maluses, so i use them to retrieve the culture values and region wealth.
Also, the first turn is winter so i think there is already penalties in place.
Also i have never played Rome (LOL) so i don't know if auxiliary units change due to reforms
If im not mistaken, if you don't research the blue techs (the ones that unlock reform units) you should be able to recruit for example, camilian units after imperial reforms?