Here it is. The tip of the iceberg for me.
I have a new gig where I need to work from my XS at home and go to the studio to do live rehearsals/recordings etc but they have an XF. I've never been in front of an XF before but have seen a lot of great things on YouTube and so on. So, my quandary is whether this is a workable thing or not? After all, there are plenty of similarities.

And here it is... The bottom portion of the iceberg. 🙂
There are indeed plenty of similarities, but there are plenty of differences, and those are the ones you must be aware of if you are to be successful.

The Motif XF can read certain data made for the Motif XS directly from the file. The XS used DIMMs for sample RAM. This was expandable, volatile memory. The XF does not have the DIMMs expansion, it uses FLASH BOARDS in addition to 128MB of built-in SDRAM. Data that was written to DIMMs in the XS can be installed on optional FL512M/FL1024M Flash Boards.

The direct file compatibility between files goes XS > XF (not the other direction). You'll need the Melas Tools to move the other direction, and there maybe some data simply not in the XS (for example, the XF has effects not found at all in the XS and not even the Melas Tools can work around that).

Just to bring this forward, I use Melas' tools and have access to Cubase 5.1 or whatever came with the XF in the promotional package.

You may want to verify the version of Cubase you have as the older the version the less likely it is yo be still supported by today's computer operating systems. Cubase series 5 is at the far end of the compatibility list, unless you are still running a computer OS from back when Cubase 5.1 was current, you might want to verify it still is fully supported by your current computer OS.

Steinberg usually offers generous upgrade paths to current versions of the commercially available Cubase (they are up to series 8 now).

That is most helpful info to be aware of. If compatibility is indeed not entirely backwards but forwards then these means I can bring my work to the XF and load it, tweak it and save it for later but can't bring it back to the XS for further tweaking. Am I getting that right?

Am I getting that right?

Yes. Time moves unceasingly forward. 🙂
The new item can read the older item's data because it is a known fact, by the time the newer one comes out.
The older cannot anticipate all the new features that were added years later.
It's basic -time moves ahead-, don't over think it.

Ok. Cubase 8 is installed and working and have Motif extensions and editors for both XS and XF. Just a small snarl with connecting via FW but nothing major.
So, here it is. I'm going to create a light, small project with the XS and save. I'll use only a sound or two from the XS that is also on the XF. When I go to the XF, will I be able to load the same project and have it come up with the XF editor and extensions? Trying not to over think it.
This is before getting into any sampling with either RAM or the Flash comes into play. Only basics.

When you open the Project while connected to the Motif XF, you will be asked if you want to REMAP the ports (do not skip this... it is much easier to simply select the "Motif XF" port when asked then to manually have to do it one track at a time later. I mention that because 9 out of 10 people do not even read the screen that pops up... but what it is asking you in theory is - "I detect that the Motif XS that was originally used is no longer connected - would you like to repoint this Project to another device..." Do so!!!

Then when the Project opens, it will be directed to the now connected Motif XF.