The 128K and 512K both use the original 64K ROMs that are incompatible with the HD20 and HFS partitions. On the 512K, the HD20 INIT enabled the use of the HD20, 800K floppy disks, and HFS partitions. You are correct that it will not work with the Mac 128K because it simply does not have the memory capacity unless it has been upgraded to 512K of RAM. Pina does say it is also possible to transplant Plus ROMs into the Mac 128K to enable it to access the HD20 without the use of the incompatible HD20 INIT. The 128K ROMs in the 512Ke, Plus, SE, Classic, Portable, and IIci all have HD20 and HFS support in the ROM, while later Macs will NOT support it, regardless of whether or not the HD20 INIT is installed. (There was a utility that allowed the SE/30 and possibly other Macs read-only access to data on an HD20, but it was officially unsupported by Apple: see the technote here.)
The larger capacity of the HD20 made it incompatible with MFS, and the way the HD20 INIT always loads HFS support, so unless you somehow hack the init, there's no way to "force" the HD20 to use an MFS file system. Apple only supported MFS on single-sided 400K disks. In systems prior to 3.0, 400K disks were formatted as MFS by default; in the early versions of System 7 (prior to 7.5), 400K disks formatted as HFS, but it was possible to force a single-sided disk to format as MFS by holding down the option key throughout the formatting process (until the formatting is complete). Support for the creation of MFS volumes was removed entirely from System 7.5, write access was disabled in System 7.6.1, and support for MFS volumes was removed altogether in Mac OS 8.
I don't have an HD20 myself, but I know a couple of people on the boards have posted pictures of their drives. I haven't taken the time to study the pictures, but my understanding is the drive used a specialized Rodime mechanism that did essentially allow it to be seen by the Mac as a giant floppy drive. Hopefully someone with one of these drives can contribute more information about the interface.
The larger capacity of the HD20 made it incompatible with MFS, and the way the HD20 INIT always loads HFS support, so unless you somehow hack the init, there's no way to "force" the HD20 to use an MFS file system. Apple only supported MFS on single-sided 400K disks. In systems prior to 3.0, 400K disks were formatted as MFS by default; in the early versions of System 7 (prior to 7.5), 400K disks formatted as HFS, but it was possible to force a single-sided disk to format as MFS by holding down the option key throughout the formatting process (until the formatting is complete). Support for the creation of MFS volumes was removed entirely from System 7.5, write access was disabled in System 7.6.1, and support for MFS volumes was removed altogether in Mac OS 8.
I don't have an HD20 myself, but I know a couple of people on the boards have posted pictures of their drives. I haven't taken the time to study the pictures, but my understanding is the drive used a specialized Rodime mechanism that did essentially allow it to be seen by the Mac as a giant floppy drive. Hopefully someone with one of these drives can contribute more information about the interface.