System 6.0.7 is compatible all the way down to the Macintosh Plus; the only
real requirements are 1MB of RAM and 800K disk drives. Your hard drive
should work fine.
The one thing to remember is that you do not want to use this external hard
drive you will be carrying around as the startup drive, because the installer
scripts for each machine is different. In other words, you need to set up the
system software for a particular machine. This would cause a problem if you
used the hard drive as the startup drive on different model macintoshes. The
only exception would be if the machine you connect it to is set up with the
same installer scripts as the hard drives system software.