I would like to use www.domain.com/squirrelmail to forward to a domain's login, instead of having to login to the main admin URL. Is this possible?
Yes, this is possible. You will have to create a folder squirrelmail in the website and then install squirrelmail within this folder. Or you use mod_proxy to proxy requests from a central squirrelmail instance to this folder. http://www.howtoforge.com/apache_reverse_proxy_ispconfig