Welcome!
The search feature is right up top
A user is the owner of 1 domain and that is also the login account.
A user can add alias domains so he can manage those as well. You could concider those dedicated domains, but managed with one i-MSCP account.
1.2.3 will be released very soon, because of this the plugins are updated.
The latest versions have had some bigger changes, including a new API version (which essentially means that new plugins do not work with 1.x)
So if you just install your plugins now you'll be fine, you can upgrade when 1.2.3 comes out.
Read the errdata inside the package or on Github, then follow the instructions: http://wiki.i-mscp.net/doku.php?id=start:upgrade:debian
Essentially this is a re-install, but all your files and settings are stored and will be used to configure the server.
You can change the FTP client with this installer too, use the -h function to see all options, look for webftp or filemanager (not sure)
Good luck with your installation!