Are there big difference in the degree of zeal among "fan, enthusiast, maniac, fiend, geek, zealot”? If Yes, what are they in order of the enthusiasm?

Language being what it is, a complete ordering is too much to ask for. Nevertheless, I think they can be roughly grouped. "Fan," "geek," and "enthusiast" are aikosha, with "fan" perhaps the least intense, while "fiend," "maniac," and "zealot" are -kyo. I would add that "fanatic," the etymological root of "fan," is in the severe category.

User867 rightly notes that some terms - I think especially "geek" and "nerd" - also describe a particular mode of appreciation. The enthusiasm of those who paint their bodies in the colors of their favorite team and stand shirtless in the crowd is spoken of differently than that of those who carefully study the statistical performance over time of that team's members and coach. The latter kind is what is meant by a "geek" or "nerd", and thus these terms can be applied to a broad range of intensity.