Those DL580's have 8 2.5" drive slots. Biggest is 1.2TB SATA, I think. You can use any configuration you want with the remaining drive slots. You'll just create a new Array (have to boot into Array Config Utility or Smart Start) and then Vmware will see the disk and you can make another datastore.
You should keep all the drives the same size in an individual array, but if you are making a new seperate array, only those new ones need to match.