There was a panel at the convention I was just at including Lou Anders of PYR Books, Chris Roberson, and Martha Wells (2 very talented authors of steampunk), the author who has read every SF book, ever, and knows them upside down Scott Cupp, and I sat in on the audience because I is no steampunk expert.
Steampunk as a term is silly and innaccurate and an accident of the proximity of cyberpunk.
Chris called it "Yesterday's Tomorrows". It's a kind of alternate history wherein that moment when technology was still tinkerable, tactile, and visceral - as opposed to electric and digital - became the mainstream. The Babbage Engine was completed, and steam was the heart of computers. Airships were made practicable, and aether - an abandoned scientific theory - is real.
however, chris calls this a form of alternate history: "Yesterday's Tomorrows". He argues effectively that it's bigger than we know.
I think Chris' term is accurate. I also suspect that outside of alternate history, what people often refer to as "Steampunk" (for instance, Martha Wells' books) are fantasy with Victoriana influences.
Thus, Steampunk doesn't exist. It's two things being shoved together, both of which are very different. 1) Yesterday's Tomorrows, and 2) Victoriana.
Many, many people disagree with me, including Lou Anders of PYR.
Adrienne, you'll do wonderful things. Pick whatever you need to use, and do the heck out of it, and don't worry about "Steampunk" defined.
One of the great advantages of SF/F is, as I learned from a DVD extra from Joss Whedon, all history and myth are human. So, when you pull influences together, you are pulling together all sorts of human things that wouldn't necessarily be together in reality, but can teach us about being human.