I asked WDC support about supported drives a while back - and there does not appear to be a specfic list. What is specific is that the drives should be WDC. Making that choice ensures that WDC Technical Support will talk to you if you have an issue.
Personally, I just added a couple more 1TB WDC Caviar GP drives and all was perfectly fine. They use the GP drives by default so that should be a safe bet.
re: the RAID. Well now, good new/bad news depending on what you really want at the end.
Firstly, when working in the RAID world the smallest physical disk typically dictates how much disk space is allowed to be used on larger drives. So if you have 2x2TB & 1x1TB, then to use then in anything other than RAID0 will mean that you effectively have 4x1TB available drive space per drive. Still with me?
If you want to go RAID5 (as I have and would strongly recommend it) - then you would be able to - but the RAID array will be built as 4x1TB drives. You won’t get the benefit of the 2TB drives additional storage capacity.
As mentioned, I went for 4x 1TB drives in a RAID5 and that has given me about 2.7TB usable space.
Now, if you don want RAID5, you have another option, dual RAID1 mirrors. so you can keep your existing RAID1, drop in the other disks and create another RAID1 usable pair. It’s fine doing this but I wanted the simplicity that RAID5 gave me and was not overally concerned about performance - bearing in mind it is networked attached storage after all.
If I were you - I’d back up what you got… (you have got a back mechanism havent you? )
Go RAID5 and leave it to tick over of its own accord…