I recently posted a thread in the conquests thread. It shows me booting a Macintosh SE off a zip disk. So you can definitely use one as your boot drive. Though mine now has a working hard-drive so that's what I use to boot it. The bootable zip disk I made has verious versions of MacOS installers on it. Something I plan to use with any additional Macs I plan to acquire.

Zip drives are only slightly slower then the hard-drives of the period and on an SE or Classic I would think the difference wouldn't be that noticeable.

So if you are worried it would take as long too boot from one as it does a traditional floppy disk...don't. They are much faster then floppy disks.