Hey Kathy,
1. You can absolutely grow the volume past 16TB in 8.1 (the aggregate will convert from 32bit to 64bit automatically as you add more drives to it)
2. There is no need to un-dedup for any reason.
3. Make sure when adding disks to the aggregate, that they're the same speed/size as the existing disks in the aggregate.
Let me know if you have any questions!