Hi Laevus, and welcome to WW.
I think the thing about not "breaking the rules" for your genre is really about not disappointing the readers who bought your book because they're looking for a certain set of satisfactions. As long as you deliver those satisfactions, then you've got quite a bit of freedom in what
else you do: what other satisfactions you deliver. The trouble starts when you start wanting to do something "other" which cuts across and weakens the things that the core of your readership want. Which is really what the command to "read widely in your genre" is all about: the more those satisfactions work their way into your own consciousness, the better and more intuitively your imagination and writerly sense will integrate the genre pleasures and the new, different stuff into an integrated whole.
FWIW, this is my take on the issue:
http://emmadarwin.typepad.com/thisitchofwriting/crossing-genres-the-perils-and-pleasures.html
Plus, the further towards the literary end of the literary-commercial spectrum you're working at, the more all these boundaries dissolve. (Though other hard-to-sell-nesses take their place, of course)