Quote by beewee: Geeklog can work with multiple installs, there's a thread in the FAQ's about it..
If you install only one theme (like the core install has) users can't choose a theme, and you can also set this in config.php. The same for comments, you can set these in config.php. I'm not sure about showing the comments on the bottom of a blog, but with some coding that should be possible.
Why don't you download the script and have a look at config.php? That will tell you a lot more.
Adsense is no problem, you can put it without hardcoding in the sideblocks or in a centerblock, and with hardcoding the templates you can put Adsense nearly anywhere you like it.