If you manually install Joomla!,as you did Drupal before, you can put it in any directory you want (including your /home/username/yoursite.tld/ directory) whether or not there are already any files or directories there (as long as there are no directory/filename conflicts). The “must be empty” requirement is a characteristic of the DH “one-click” system, I assume to prevent any existing directories that might have the same names as a directory Joomla! requires for m being overwritten.
As there are actually no directory name confilcts between these two applications, alternately, you can simply “move” the existing “WordPress” directory (home/username/yourdomaintld/ri?) to a location “above” your “web directory” (for instance, to /home/username/ri), temporarily, use the one-click installer to install Joomla!, and then “move” the “WordPress” directory back to it’s original location.
Either of these scenarios will work just fine, and is a much better solution than using re-directs to make various directories reachable via various urls.
If you really want to rely on the “one-click” installer, avoid the issue you are dealing with now, and avoid potential conflicting .htaccess files while using various one-click packages, in the future you might consider using your “base” web directory for your “main site” (like Joomla!) and using sub-domains (free at Dreamhost) for things like your blog (blog.yourdomain.tld) and gallery (gallery.yourdomain.tld), etc.