1) OSX will not send any sequence when you press C-6 in Terminal.app. You have to press C-S-6 or C-^. I've never had this issue on Linux. Either C-6 or C-^ will work.
2) If you launch Vim with "vim file1 file2", the Ctrl-6 buffer switching will not switch you between file1 and file2 until you manually do it for the first time in that Vim session.
3) The Ctrl-6 switching is frame-dependent. The "previous buffer" is maintained locally between window splits or tabs in Vim, so you could have a vertical split that allowed you to easily swap between 4 separate files (i.e. 2 per side).
C-^ (aka Ctrl-6) switches back to the most recently used buffer. That is, if you are using two buffers, you can just flip back and forth with C-^