Some drives will be set to SCSI ID 0 if there are no jumpers on the ID selector - there is often a label on the drive that explains the various jumper positions. You could always take jumpers off the original drive if you need more. It may be that the original drive is also set to ID 0, as that is the conventional ID to use for the main internal drive, which could explain why it didn't work when you tried both together.

You could use the Mac OS 7.5 Disk Tools floppy (this should be around for download somewhere), which contains a boot system to start up from and one of the Apple formatting tools (HD SC Setup is the older formatter made by Apple; Drive Setup the newer). Be careful if moving files through a PC - Mac applications must be inside an archive like .sit, .sea etc if they are to survive intact.