the -6 stands for 6/16ths which is 3/8ths. You can use standard 3/8ths fuel line with some -6 fittings (depends on the outer diameter of the hose)...
I HATE with a capital HATE, SS braided. I always use the Nomex or Nylon braided for my projects (depending on application). Much easier on the hands
... If I need something for extremely high pressure I use the TFE hose that is SS, but that is usually only for power steering lines and stuff over 1500psi (which is only really PS).
FWIW I think Nylon braided is NHRA legal now too. So it passes tech. I know that was an issue before.
The plummer's tape on the oil cooler lines is a good idea. Too bad I would probably have to pull my engine to get to them. I would use the oil/gas resistant plummer's tape though, it is usually pink instead of white. It would work better in the oil cooler application. AN fittings are supposed to seal though, I mean they cost enough...they should seal. It is only after everything is installed that you find the leaks and then it takes exponentially more time to fix/tighten.