Hi, welcome to the forums
Managing multiple domains on DreamHost is about as easy as it gets. Each domain, or sub-domain, has it's own web accessable directory and you can configure separate email accounts etc. for each.
You can configure all your domains under the one user, or place individual domains under the control of different users. When a user logs-in via FTP, they will only see the files pertaining to the domains that they control.
This can be handled a number of different ways. You could create a sub-domain of your 'release' domain and use that for testing, or you could create a free dreamhosters.com sub-domain and use that.
Unfortunately no, but I don't think it would be too difficult to install Drupal manually.
The applications that DreamHost currently supports via one-click-installs are as follows;
PHP4 or 5:
WordPress Weblog (v2.0.4)
phpBB Forum (v2.0.21)
Gallery Image Album (v2.1)
ZenCart Store (v1.3.5)
Joomla (Mambo) CMS (v1.0.11)
activeCollab Collaboration (v0.6)
MediaWiki Wiki (v1.7.1)
Advanced Poll (v2.03)
WebCalendar Calendar (v1.0.4)
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