SCSI drive question

I have a server with three Western Digital 18.3 GB HD as Raid 5.
I need to add some more disk space but the hard drive were listed as legacy equipment on WD web site. Do I need to match the Hard drives exactly? I mean can I put in Two more hard drives that are not exactly the same but were SCSI and were the same RPM of the first three.
and am I correct that when I add hard drives that all I need to do is plug them in and use disk manager to reconize them?