1) There is a beta version with a smaller bar. Fixing the zoom issue is not possible until Mozilla adds a toolbar API.
2) Download speed is in the beta, other stuff is in the tooltip but could be added to the bar in the future.
3) Might be possible, I'll have to look at the API
4) There is already an option to auto clear downloads after a certain amount of time.
5) I'll probably add more themes, and customization in the future
6) If seen this, but I haven't been able to reproduce it consistently to fix it, I'm not sure if it is a bug in the addon or Firefox.