Thank you, @scrawly! I'm so glad you like her! ^.^

Yeah, every composition can be created using different rules depending on how much elements you have, which one is the main, the others must be subordinate to him (I think that is called Principle of unity and subordination). There're a lot of principles, the same goes when you use color palettes... you have to keep in mind which colors are analogous, complementary, etc., etc...
It's not wrong to not think about that stuff - sometimes I think more like a designer rather than an artist. ^^'

About the topic of using references... There's a lot of speculations how much you can use a reference (do you have a permission to use it in a first place) and where is the line between refs and plagiarism. I always use reference photos, because I'm not that good with human anatomy and colors, but always try to add something from me, because my perception is different from others (and that's the greatest thing - we are all unique and see the world not as it is, but based on our mental patterns, understandings, perception...)

But! Some may say that I've just used the woman from the photo... and that'd be their opinion... ;)