Almost any SCSI CD-ROM will work in your Quadra 950. Even SCSI CD burners should work but I doubt you'll be actually able to burn CDs.

Non-Apple CD-ROMs will require a different driver than normal, but there are a couple of them that work with almost ALL drives. I have installed A/UX with a non-Apple external SCSI CD-ROM on a Quadra 700. I had to install a different CD driver on the A/UX boot disk first, then it worked fine.