A disk image of the 10.6.3 Server retail DVD works. I just tried it again with Fusion 11.0.2. (I got as far as initial setup questions after install, then shut down and deleted the VM since I don't need another one.)
I recall that with my previous installation of Snow Leopard Server in a VM I tried using the DVD directly and it was not recognised by VMware Fusion, therefore I agree with dlhotka that the disk image is a necessary step. You don't need anything newer than the 10.6.3 Server retail DVD, which is good because that was the final release.
I don't have the original 10.6 Server retail DVD so I can't test whether that works.