I have used both an internal JAZ drive, and an external ZIP drive. The ZIP was limited to ID 5 or ID 6. As can be seen from
this, my ZIP had been set to ID 5.
Unless currents ZIPs don't have the same limitations, its still ID 5 or 6.
For your CD-Rom you are free to use any unallocated IDs. ID 7 is usually the SCSI Host Addaptor itself.
Be sure to check with other A2091 users as that Host Addaptor predates CD-Roms. I havn't used one since 1991, using the 32Bit SCSI Controller in, first a GVP accellerator, then a Blizzard accellerator.
BTW, it is possible to use TWO SCSI Host Addaptors at once. I had first used the ZIP on a GVP HC+ card's external port untill i obtained a Db50 - Db25 cable.
Zip Tools allows more than one scsi.device name to be set.
Both Amiga-formated and PC-formated ZIP Disks can be used with the correct Drivers (mountlists). Something such as
Fat95 is needed for long disk & file names on PC-formated ZIP Disks.
ZIP Disks are sold with either PC or Mac formatting.
ZIPMount by Stefano Cairoli is helpful for use with PC & mac formatted ZIPs.
Zip4mat by Lorence Lombardo makes preping & formatting ZIPs easy, and to Re-PC Format an Amiga Formatted ZIP!