As far as I can tell from my disk/manual sets, those systems came solely with "stock" System Software disks. I cannot find an HD20 Init on any of mine, Systems 6.0.1 through 6.0.8.
Yes, they shipped generic install disks that could be used on any Mac. At the time, system software updates were 100% free, so the copy of 6.0.7 you got with your IIsi could happily be used on a Mac Plus. (Well, that may have been a bad example, since the IIsi came with HD disks... But you get the point.) The installers on all of my 'system-shipped' System 6 disks include the "Minimum System for xyz" for every machine that was available up to that time. So, yes, my IIsi-shipped HD 6.0.7 disks do, indeed, have an option to install a Macintosh Plus system.
Yes, they shipped generic install disks that could be used on any Mac. At the time, system software updates were 100% free, so the copy of 6.0.7 you got with your IIsi could happily be used on a Mac Plus. (Well, that may have been a bad example, since the IIsi came with HD disks... But you get the point.) The installers on all of my 'system-shipped' System 6 disks include the "Minimum System for xyz" for every machine that was available up to that time. So, yes, my IIsi-shipped HD 6.0.7 disks do, indeed, have an option to install a Macintosh Plus system.