I have a question about introducing Area of Recruitment to my own personal modding project.

I downloaded the "Additional Units Pack." And made the Celtic Swordsman recruit-able for several factions. The problem was it was way too recruit-able for these spread out factions, and I wanted to make the unit only recruit-able in the many cities the Celts would have settled around. When I tried to make these cities hidden resources and add them to the build requirements, there turned out to be problems with almost every city except the first one I added to the list. Is hidden resources only usable up to one region?