Frank wrote:
Does the S90 XS/S70 XS EDITOR VST setup detailed here for Cubase 8 Pro apply to the S90 ES as well?

The S90 ES predates by several years a time when the Editors could run as proper VST synths. In fact back in 2005 when the S90 ES came out, the S90 ES Editor for Voice mode was a separate Editor from the one handled Multi setups. The current crop of Editors have been integrated into one single program that does it all.
The S90 ES had a separate Voice Editor - that essentially handled all Voice editing. The Multi Part Editor handles Mix setups and is able import Voices from the Voice Editor. They required a host application called 'Studio Manager' to run on your computer. The Studio Manager negotiates MIDI connectivity and manages files (storing and restoring your exact setup; both Midi port setup and your sound libraries). You ran this along side your favorite DAW... At one point it was even integrated into Cubase (but current product uses Steinberg's VST (Virtual Studio Technology) protocol to integrate the External Hardware into the computer environment.)

If not, where is the best explanation of utilizing the S90ES best in Cubase 8 Pro?

There can be no complete answer to such a question without you telling us something about what you want to accomplish. Cubase Pro 8 does not really care what synth you are working with... Setup will pretty much be the same. The S90 ES can communicate with your computer via USB for MIDI. You don't say, but if you also want to record audio you will need an audio interface and a quality set of monitors. Whether or not you can run Studio Manager and the Voice and Multi Part Editor along with Cubase Pro 8 will depend a lot on the computer and computer operating system you are running.
Please recognize that it is 10+ years since the S90 ES came out, and coming up on 7 since the S90 XS replaced it. I mention that only because it's not so much that the software will not run on today's computer OS's, it's that no one can guarantee that there will not be some features that will simply no longer work. They don't say it's 'incompatible' always, some times it simply becomes 'non-compliant' or if it does work, no one is actively supporting it after a certain amount of time.
That said, many folks are still successfully running the older software editors on the latest operating systems. Just go into it with your eyes open, and with some expectation that somethings may not be up to it... Simply because of the length of time and the number of changes that both PC and Mac platforms have gone through.

With my Yamaha s90 ES I want to use multiple midi tracks within Cubase 8 Pro simultaneously playing multiple sounds i.e. piano, drums, pad sounds etc.

That should be no problem. Worst case you will absolutely be able to this!!!
Whether or not the Editors can run on your yet unidentified computer is the only question. In a case where you couldn't use the Editors, you could still accomplish your goal. The Editors add a bit a elegance in managing data; but certainly you can operate with out them.