Based on the description for hierophant taboos and abilities, the spirit is the devout worshiper, the medium is just accessing divine power through the spirit's faith. The medium should probably be within a step of their alignment, but I don't think they have to be a worshiper.
You choose which spirit you contact each day. For my champion spirit, I have a favorite that I contact each time (he provides proficiency in bolas). We've been partners for awhile, and we're used to working together. But when I need a Trickster, I'll contact a different local spirit each time depending on which skills I need.
Basically, whatever works for you.