That's a cracking first pen Gavin. There are no rules about the shape of any pen, nor pen police to tell you off. Common sense, ergonomics and aesthetics need to be high on the agenda though, as some shapes will look just wrong or will be impractical from the point of view of actually using the pen.
Personally I turn slimlines so that they're parallel, which entirely subjectively, I think looks better that way. I've seen others though that are really quite fat indeed - quite the opposite of slim in fact. There's no right or wrong, but I do wonder just how easy the fat ones are to hold and to write with.
You'll get used to how far down to the bushes you need to turn. The material you're turning will have quite a lot to do with it, as will the extent to which you need to pretty up the tooling with abrasives. Soft pine will abrade much more readily than say an exotic hardwood, so turning both of those down to the same extent may possibly result in the exotic hardwood being on target by the end of the process, and the soft pine being under sized. Just experiment and enjoy the journey, that's my advice. You'll pick up masses of knowledge along the way with the benefit of experience, and even with a zillion pens under their belts, everyone has a failure from time to time, so don't be afraid of that either.