Hi Wil,
Yes. You're right. It would be a temporary downgrade to install a Snow Leopard Server on an older Mac. Once installed and patched with Apple's latest software updates, then the Snow Leopard Server guest virtual machine would be copied and run on a modern Mac with the latest versions of OS X and VMware Fusion.
The VMware Fusion license says, "You may install and use VMware Fusion for personal, non-commercial use on any Apple-branded products running Mac OS X ("Mac Computer") that you own or control. - See more at: https://www.vmware.com/support/fusion/faq/licensing#sthash.k3ApXLEg.dpuf"
Reading between the lines, I assume that it is okay for a user to run both versions of VMware Fusion on two different Macs that they own or control. The question is how to download older versions of Fusion and get a license to run it.