HDD recommendations for Readynas NV+ v1

You can mix capacities, but you won't see vertical expansion until all drives match (The NAS would treat all drives as 1 TB) Also, given the age of your equipment it is likely you'd need to do a factory reset to get 4K alignment anyway. Otherwise performance will suffer.

You also might need to update your firmware. What are you running now?

Since you are out-of-warranty, there are no support implications to straying from the HCL. Personally I'd pick the WDC WD10EFRX (or the WD20EFRX if you want 2 TB for ~$30 more). Usually I'd suggest going with the 2 TB drives, but in your case your storage needs do not appear to be growing very quickly. So it is a toss-up. You might want (or need) a new NAS before you need larger drives.

I'd avoid the newer "green" disks (WDC or Seagate). The NAS disks from both vendors (WDC EFRX and Seagate NV000) seem to do better in NAS and have better warranties. Seagate doesn't have a 1 TB model though.