No, Apple will not officially support the installation of half-height hard drives in the Workgroup Server 8550. The drive carrier in the Workgroup Server 8550 computer will only support the addition of a third-height hard drive.
NOTE: Even though it _may_ be possible to physically fit a half-height hard drive in the bracket itself, the screw holes which mount the drive to the bracket will not match up. If you do somehow manage to install this type of drive, Apple will not support its use in your Workgroup Server 8550.