The device (hardware + boot loader) should be able to handle most 6TB drives without problem. I'm not sure if the Cloudstor firmware can handle them or not (having never used it) but I believe the regular firmware could and would suspect that it can.
I believe typically only the Terastation device's firmware has the AD/Domain features but am again unsure about the Cloudstor firmware.
One way to work around any shortcomings with the device's firmware would be to replace the firmware with a full Linux install. Information about how to do that can be found here:
https://github.com/1000001101000/Debian_on_Buffalo