1. Galaxy does not download and activate installers - it downloads games in pre-installed state and then runs the install script if required by game (some games do). If you want to download the standalone installer, you can do this via MORE -> GET BACKUP & EXTRAS option for any game.

2. You can manage download folder (that's where games / updates are temporarily downloded), extras download folder (goodies) and game installation folder (final path for storing games) in settings of the Galaxy - just click the cog icon in the bottom of the sidebar (on the left). The game, once downloaded, is not kept in the download folder though, so no space should be wasted.

PS: If you have further questions, I'd recommend discussing this in the GOG Galaxy dedicated thread at http://www.gog.com/forum/general/gog_galaxy_beta_2