If I want to buy a lord that needs only 1 culture, I should use only that culture, cannot add another to complete the amount required?
Also, a lord requiring 3 cultures must be purchased using only 3 cultures or can I purchase it with say 4?
The demand for cultures/races is a strict one. You can never deviate from it. You can, however, add pearls to your ally cards in order to make up for lower value allies. (As the response above demonstrates).