Yep no problems at all booting from an external hard drive. 
But yes, Pluses never had an internal anyway... booting from floppies and running off floppies was the normal manner of operation back in those days. Hard drives were very much a luxury item... First Compact mac to include one standard was I believe the SE FDHD. Depending upon the program, you either had to do the old switcheroo between floppies to load the program to memory, or some programs, were on bootable floppies with a cut down system file.
The System 6 disks arent Installer disks per-se, more a set of disks with most of the tools you need to do things, System Tools and Disk Tools are the ones you use the most.
But yes, Pluses never had an internal anyway... booting from floppies and running off floppies was the normal manner of operation back in those days. Hard drives were very much a luxury item... First Compact mac to include one standard was I believe the SE FDHD. Depending upon the program, you either had to do the old switcheroo between floppies to load the program to memory, or some programs, were on bootable floppies with a cut down system file.The System 6 disks arent Installer disks per-se, more a set of disks with most of the tools you need to do things, System Tools and Disk Tools are the ones you use the most.