Yes, you only need a FW connection to the PC to be able to configure the settings and the routing of the FF800.
Then you could theoretically operate it standalone, BUT ...
This will work fabulous for analog ports, but for Mic Preamps you want to be able to enable/disable Phantom Power
and adjust the Gain. Therefore you should keep the FF800 connected to the PC via FW for pure operational reasons.
But tbh .. I wouldn't start now with Firewire on top on the PC, esp as FW is a dead horse on Win10.
There are legacy drivers available .. but I wouldn't do it.
It is also a little bit more uncomfortably, having to deal then with 2 separate TM FX instances and driver settings dialogues.
The ideal solution would be IMHO, to add another XTC, because then you can remote control BOTH XTC
as AUX device and store all settings digitally in the one TM FX instance of the UFX II.
With 2 XTC you would have 2 MADI capable devices.
In the future I would sell UFX II, get UFX+, then you have maximum comfort, minimum cables.
UFX+ costs only €400 more compared to UFX II.
And you have 64 channels more for 44.1/48 and 32 channels more for 88.2/96 for high quality recording.
Alternative A)
If you have 4 analog input channels free on the UFX II you could also add a Qadmic:
http://www.rme-audio.de/products/quadmic.php
https://www.thomann.de/de/rme_quadmic_ii.htm €581
This has the advantage, that the 2nd ADAT port stays free for
- any future 8-port extension (8-Port ADDA converter or another Preamp)
- if you want to perform high quality recordings @88.2/96
Alternative B)
Get an Octamic II
http://www.rme-audio.de/products/octamic_2.php
https://www.thomann.de/de/rme_octamic_ii.htm €1299
The ideal setup for you would have been the UFX+ and then to connect as many XTC as are required via MADI,
where the MADI cables can even become up to 2km long between the devices.
MIDI over MADI would have saved you MIDI cabling for remote control as AUX device.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13