There are a couple of possible approaches to this problem.
It would be, of course, possible to compile a different kernel for installation that included support for both of the SCSI cards/chipsets you need for the server and use it for a Slack boot floppy along with the Slack root disk and the CD or even to burn a new install CD including that kernel. This approach is not likely to be the simplest for you however.
What you should keep in mind is that this is a problem with installation and is not that likely to continue to be a problem after installation. We can start by asking the question, which driver do you need more for the installation? It would seem that you need the CDROM to read the install disk and the RAID array to have the system hard drives available. However, it would be possible to use another device on the system to serve either function. The next factor that enters in is what you want the end result to be. You want the computer, I would imagine, to boot from the RAID array, so that makes the driver for it the more important one. Then you just have to ask yourself what alternatives are there to installing the system from the CDROM that is on the server. It is possible to do a network install or to copy the installation files to a partition on the hard drive array using another OS. You should never, however, overlook the simplest approach. In your case this may be to hook up an IDE CDROM just long enough to get the system installed on the machine. Once the system is installed, you can most likely load the CDROM driver as a module during boot rather than having it built into the kernel. After this, there will be some configuration adjustments to point everything to the correct CDROM, but they will be minor. If you wish, or if it is necessary, you may wish to compile your own kernel for this machine once the operating system is up and running.