I can confirm that you can indeed add widgets, if there is space to do so. Coming from 2 decades of using Visual Studio to designing gui's I find Glade to be minimal in functionality, but it beats hand coding :-)
It is worthwhile starting here for the layout etc. Wiring python code to the actions/signals is probably best learned via other sites relating to gtk and python. Or looking at Tormach code. Perhaps start by tinkering with something simple such as file_chooser.glade and its related python code. Still, it won't be a 10 minute learning curve afaict
gerrit