Hi Baz, I think that I can help. Bonsai are supposed to represent what trees COULD look like in nature--and these trees, while no two are exactly the same, mostly fall under the "informal upright", "slanting", and so forth. However, you're right in the fact that there could be some "undefined" styles; I think that you do not have to shape a tree EXACTLY the way an "informal upright" would look. I myself want to train some trees that do not have any set style.
hope this helps! Take it with a grain of salt, seeing as I'm still kind of a newb too.