Is it wise to go for same brand/model for all the drives in a NAS, or is it ok to mix? What about mixing between different raid groups?
I think for simplicity's sake, it is easier to keep the drives the same size, geometry, and speed. It might save you some headaches later.
A possible argument for heterogeneous/mixed RAID arrays is to reduce the chance of multiple disk failure - probably fatal for your NAS. If you have a NAS with identical drives (homogeneous RAID array) then it seems logical that you are increasing the chances that more than one drive will fail around the same time. However, this thinking does seem to be based more on speculation rather than hard fact.
(Overtime, keeping all drives identical in a NAS can be a tricky/expensive task. If one drive fails and that same drive is no longer available, do you begin the process of replacing all drives in your RAID array?!)
However, I believe homogeneous RAID arrays generally provide better performance.
I have a Netgear ReadyNAS NV+. No where have I seen it recommended that all drives should be the same. In fact most users seem to run it with different drives. The only recommendation, however, is that the drives used should be on the hardware compatibility list. (Or check the community list.)