Most SCSI RAID controllers will accept different kinds of hard drives. The hard drives should match in capacity points and rotational (RPM) speed. At best, all drives in an array will be identical--at the same firmware revision level.
RAID can be used with any size hard drive. The smallest capacity drive will determine the largest logical volume size for all drives in the array.
Whenever possible select drives from an approved vendor drive compatibility list. This ensures that the hard drive is tested, and should function reliably with your SCSI RAID controller. Untested configurations 'should work' too. But for best results, select only tested and compatible drives for your SCSI RAID controller. Most SCSI controller vendors publish a hard drive compatibility matrix on their Web site.