The question title says it all - are all TIFF file headers and IFDs 8bit? And the image data itself can be specified in a different bit order in the IFD, right?
AFAIK, 8-bit and 16-bit in TIFF file refers to the image pixel data contained in the file itself. Headers and IFD are the same irregardless of what bit-depth the TIFF file is in.
The first 8 bytes forms the header for the TIFF file, and the IFD contains 12 bytes.
EDIT : You can find out more information about the structure of a TIFF file here.