rules? the only rule i know is shooting for the subject according to the final purpose or use of the image. talking about f-stops and lighting is really pointless, because exposure settings are going to be dictated by personal style, subject matter and the final depiction thereof. if that's what you were expecting, then you can stop reading [img]/ubbthreads/images/graemlins/smile.gif[/img]
fashion photography is a pretty broad category. but to oversimplify things, the subject of fashion photography is ... fashion. the clothes. you're shooting the clothes, not the model, because it's the clothes you're trying to sell, and not the girl wearing them. even with editorial fashion, where you're selling the "feeling" or image of the clothes rather than the clothes per se, you're still selling the clothes. the model is just something to put the clothes on.
let's go with beauty photography. this time, substitute the clothes for makeup or lotion, or some other beauty product. editorial beauty? you're selling the feeling or image of that particular beauty product. again, NOT about the model. she/he is just a canvas to put the product on.
okay, so let's say you're shooting for a model's portfolio. it's all about the model this time, right? nope. it's about showing how well the model can sell clothes or beauty products or whatever other product as well as the image or feeling associated with them. again, nothing to do with the model per se.
flip through fashion magazines. real ones. not maxim. not stuff or fhm. pick up vogue, w, bazaar, surface, i-d, flaunt, or zink for example. study the photographs and editorials and see if you can understand why it was lit that way, or why the model's pose is like that, or why the depth of field is shallow or not. what stands out in the images? what is the unifying theme in each editorial? what are they selling in the photographs? can you pick out the key element/s of a particular article of clothing? what feeling are they trying to project or associate with the clothes? with beauty shots -- why did they pick that model? study the facial/bone structure that makes a good beauty model.
if you can understand the above... you can shoot fashion. if you know the "why", you can do the "how".
there's a few gg members who know much more than i when it comes to fashion: chung lee, eric striffler, christian behr, and jerry avenaim immediately come to mind. i've learned a lot from them just by reading their posts and critiques of not just my work but others' work as well.