I personally have an Apollo Twin Duo and Firestudio 2626. I use mine as an additional 8 ins via ADAT into the Apollo. You simply have to preset your I/O matrix so that ADAT 1-8 mirrors Analog 1-8.
One thing to note is that you would be fixed at one sample rate (i.e. 48k). ...UNLESS you're connecting to an interface that also has an ADAT out (which Apollo Twin does not have).
1. You can sync the Firestudio2626 via ADAT. You do not need to have the firewire cable connected to a computer.
2. Optionally, you can connect via firewire to sync the Firestudio2626 manually in the Firecontrol Mixer...
3. Or create an aggregate device with your main soundcard, so it will automatically switch.
I'm on a mid 2015 MacBook Pro, OS Sierra, all of my peripherals go thru a thunderbolt 2 hub.