I believe ZFS works best when having direct access to the disks, so having a md underlying it is not best practice. Not sure how well ZFS handles external disks, but that is something to consider. As for the drive sizes and redundancy, each type should have its own vdev. So you should be looking at a vdev of the 2x6TB in mirror and a vdev of the 2x12TB in mirror for maximum redundancy against drive failure, totaling 18TB usable in your pool. Later on if you need to add more space you can create new vdevs and add them to the pool.
If you're not worried about redundancy, then you could bypass ZFS and just setup a RAID-0 through mdadm or add the disks to a LVM VG to use all the capacity, but remember that you might lose the whole volume if a disk dies. Keep in mind that this would include accidentally unplugging an external disk.