"Can I set this up to have 2 tb capacity if I don't care about redundancy? " - Yes then setup a RAID0
"Can I add a 1.5 tb drive to make it 2.5 tb, or it's artificially limited to 1 tb?" - Yes but its not supported on this board
"Or it always has to be setup as a RAID, whether RAID 1 or RAID 0?" No, you can use it as two individual disks if you want (no RAID)