I don't think you can record mono on tracks 5 to 8. 5/6 is a stereo pair, as is 7/8.
Maybe you could record mono tracks on 1 and 2, or 3 and 4, then move them as a stereo pair to 5/6 or 7/8, but this would probably only be useful if you wanted to record 8 discrete tracks for later transfer to...