related to SHR: yes, you can add so small HDD as is smallest in array = in your case 1 TB.
If I understand when you add to slot 4 next 1 TB HDD (expand SHR volume) you will have: 4x1TB in something like RAID 5 (that means 3 TB usable space) and thank to SHR also 2x1TB in something like mirror (means 1 TB usable space). By my mind final usable space will be 4 TB (or more precisely something above 3 "real computer TB" (correctly TiB) ).