1) Goautodial is NOT the software you are talking about. The software is called Vicidial. Goautodial is merely an installer for Vicidial. Admittedly it has some interesting reports and some automation, but the "Dialer" software is Vicidial, not Goautodial.
2) If you install with Vicibox instead of Goautodial, clustering is automated and there is NO limitation aside from hardware (which is why clustering becomes so important, you can just add more hardware).
3) Vicibox always installs the Latest Version of Vicidial. The Vicidial Group is continually moving the Vicidial Core towards Multi-Tenant. So if you want Multi-Tenant, working with the Latest version is always the best idea. Goautodial only works with the version of Vicidial that was "out" when that version of the installer was created. So with Goautodial, you do not get the latest version of Vicidial.
4) Your best bet would be to install from a Vicibox.com iso installer into a Virtual machine and use the available controls to put two clients on the server (with one agent each, and one call max each) and see if they can be blocked from seeing any information about each other. We haven't tested recently, but last time we did the Multi-Tenant feature was good enough for most instances. Obviously you can't use virtual in production, but you can certainly use it as a testing tool to verify functionality before your full buildout. Just remember to fully load-test your final buildout before announcing any launch dates.