RE: "How UDEV works in RHEL AS3"..... I suppose it works the same way as in any distribution. I can't be specific to RH as I use Slackware, but I am assuming that UDEV is UDEV, regardless of the Linux version. To be vague and simplistic
the computer saves the /dev directorys contents at shutdown, so that any special or weird human-made nodes are preserved, and upon reboots, the machine detects all the hardware, drives, devices, etc, and creates the needed nodes for them, plus any human-made or weirdo nodes.
UDEV also uses (depending on the system) a RULES file (or several of them) and ALIASES to create special device links.
As for why exactly RH is creating a dev-link to hdc instead of scd0, I have no idea, but the CDROM device is *probably* not an actual scsi device, but in your case, you are needing direct native-scsi functionality to get the drive to do what you want it to do. ATA/ATAPI and most similar optical drives use the scsi command set internally anyways; it's just 'wrapped' with something newer (ATAPI) unless the drive is very old or very rare.
In any case, just delete the sym-link you don't want, and make a new one
and it should remain in place across reboots and you'll forget all about it! The man page for the ln command gives details, but in case you don't know, you'll want something like:
ln -s /dev/new-link /dev/scd0