Since the Plus was introduced with System 3.0 and Finder 5.1, that's the earliest System you should use with SCSI. It is unlikely an earlier System or Finder would be aware of SCSI, though the Plus will boot down to System 1.1. However, System 3.1/Finder 5.2 quickly replaced 3.0/5.1 within a month. The most widely accepted and stable system in 3.2/5.3 which came out 6 months after the Plus, and you should be fine after that up to 7.5.5.
There is a very slight possibility you have a Plus with original or Rev. 1 ROMs which can cause problems with SCSI. If your serial number is greater than 1986 you'll have nothing to worry about.
The Plus ONLY uses standard HFS. However, unless you use an Apple certified drive, you may have trouble using some of the earlier HDSC Setup utilities. Also there are size limitations on drives before System 7 and with System 7.
If you plan on using a ZIP drive you will need the Iomega 4.2 driver in order for it to boot the Plus. That also brings up some questions about formatting with third party utilities to be startup disks. Having never really used an external HD on a Plus, I merely pose the question. I do know that formatting third party drives was always a headache on the early Macs.