An older Ardour discussion that includes ADAT is found here:
https://discourse.ardour.org/t/focusrite-18i20-and-octopre-adat-connection/105897

Kernel with better Focusrite support is found here:
https://github.com/geoffreybennett/linux-fcp
edit - if you got 6.14 or newer kernel it already has the drivers it seems:
Upstreamed Big 4th Gen Scarlett support Latest This release updates the FCP driver (supporting Scarlett 4th Gen 16i16, 18i16, and 18i20) to the version accepted upstream in 6.14.

You shouldn't have any Linux-related issues with it, ADAT will only be used to communicate between the Scarlett and Saffire.

The only issue you might run into is clock sync: If your Scarlett is your main interface (connected to your computer), and you are running an ADAT cable from the Saffire to the Scarlett, then the Saffire won't have a clock to sync to, so you'll need to run a second cable - e.g. another ADAT, Word Clock (BNC), or something to send clock sync to the other device.

If your second device isn't synced you'll probably hear pops and cracks in the audio coming from it. And, if you absolutely can't get another cable for whatever reason, you can switch the clock master to the second device in your DAW or on the device itself.

Basically, what tasankovasara said, but with a few more details, mostly to underline how important clock sync is. And my experience is the same as theirs - a cable for sound, a cable for clock and it just works.

Also, if you go into a store and ask for ADAT cables and they look at you like you're from the moon, they're also called Optical Cables.

I have basically this setup. Word of warning, use bnc word clock. Don't trust adat word clock. I dont care if people say it works fine, its way too risky and plenty of good interfaces have wordclock, if it doesn't, its not worth getting.

Also, adat cables suck ass and are highly fragile. If you dick around behind your stuff and it gets jostled a lot, watch out for problems. If you set and forget and keep the dust off your equipment theyre generally fine.

I learned this from years of real world experience!

I've ran a 4 in, 4 out ADAT-CAT5 snake thingy on Linux, so 32 channels in and out. The remote end was synced via Word Clock with the Linux box providing master clock. RME RayDat for a soundcard, RME converters in the remote rack. Worked 100 % flawlessly, I even did live sound on it. ADAT is a 'just works' thing, go for it. You just need to understand that one device needs to be the clock master and others follow that.

I have no idea what any of those things are. Which is not to say, you shouldn't post this here, but a bit of context would be nice.