I have a copy of Toast 3.5, I think it is, that installs an Extension allowing my older machines to see and use an external SCSI CD-R. I don't think your CD burner is going to work on a Quadra under 7.5 without such third-party software.

I have, however, used Toast on a Quadra 840av running System 7.1, where it works just fine, so System 8 is not essential.