Rip,
The CNC slave cylinder uses a pipe-thread on the cylinder itself, as I believe do most aftermarket mastercylinders. But I'm pretty sure it is delivered with an american flare adapter fitting already installed. Not sure about the particular MC in the SACO kit - never been able to find out exactly what or where it came from. You should definitely use a steel line from the MC back to near the slave, with a flexible hose section between steel line and slave since the slave moves each time you push the clutch. Be sure to secure the steel line at the transition to flexible, keep the hard line stationary. Adapter fittings are readily available so you could switch between types of fittings/hoses/flares just about anywhere you want, but for ease of assembly I'd stay with one or the other (as much as possible). There's also AN, which you may want to use for the flex line portion since various lengths of flexible AN SS hoses are also readily available and inexpensive. I've found the steel hard lines readily available in both metric/bubble and american flare in a variety of lengths at the local auto parts stores.
Just to give a "for instance", I was originally planning on plumbing my brake system using a Porsche MC, metric hard lines (that match the MC flare), then going to AN adapters, flexible AN SS hoses, then AN to pipe and finally into the pipe thread Wilwood calipers. For the rear line include another set of adapter fittings to run the proportioning valve - talk about adapter city!! Due to other changes I've since decided to to run a Wilwood balance bar brake pedal and dual MCs and keeping it all American and AN. Careful planning is the key....
Jeff