Zone of recruit can be easily implemented using hidden resources, but is there a limit to number of hidden resources you can have? Can I add any number of hidden resources, or will it mean I'll have to remove trade resources etc.?

Another question I have is regarding mines. I'd like to have several types of mines: salt mine, copper mine, gold mine and silver mine. Is it easy to add several types of mines? in the export_descr_building.txt I can't find anything controlling need for a certain resource in order to construct the mines. Where do I look?