I removed my original hard drives and bought SCSI2SD units from Inertial Computing - a 5.1 internal model and a 5.5 external model - and they worked out of the box on my Color Classic and Color Classic II. Later, I went in with the tools to change the SCSI ID and things but mine worked out of the box. I used balenaEtcher in Windows to copy a 1GB System 7.5.5 minimal boot image I found online to the MicroSD Card then booted from it and did an install full installation from CD-ROM. I did notice some weirdness about which MicroSD card I used - the SCSI2SD adapter seemed to be a bit particular for me or else balenaEtcher was flaky. Either way, all works for me now.
Good luck.