Yes, you can boot offa scsi zip drive....if your bios supports it.
Go into the bios and set it to boot off of a scsi device first...then pop your bootable zip disk in the drive and you're good to go.
Also, if you're bios doesn't support scsi booting...disable booting offa the HD and then go into your scsi card's bios and set the ID of your zip drive to be bootable.
As far as OSes to boot...i'd suggest just good 'ol DOS. If you're into linux, i'd go with zipslack from slackware. It's a distro of linux that specifically designed to run offa zip disk.