You can use any IDE HDD/CD-ROM/DVD-ROM/Writer on the IDE bus and you can use any SCSI HDD/CD-ROM/DVD-ROM/Writer on the SCSI bus. With the appropriate cable adapters of course and proper termination of the SCSI bus.
If you want to use a HDD bigger than 4GB, make sure that either it is connected to the Blizzard and uses one of FFS V43 (patched with TD64 patch), SFS or PFS3, or that the boot partition is inside the first 4GB of the drive and all patches comming with OS3.9 are installed properly.
OS4.0 will not run on that computer because it does not have a PPC processor.
Bye,
Thomas