Hi lone_warrior
For the primary server, I would be considering using RAID 1 only. Write speed to disk is more important for the primary server. Given the information on what you are protecting the NetBackup file system should not grow beyond 500GB - this file system should all be easily accommodated with the OS volumes on the server's boot drive (mirrored in hardware).
For the media servers, as you are using a DataDomain for backup storage, these do not need to be sized much beyond the OS requirements (more memory is better of course -especially if you add MSDP to the media servers).
I would consider adding to the media servers enough disk to create an Advanced disk pool to use for the NetBackup catalog backup - this may need to be 2TB or so (1 or 2 full copies, plus incremental nbackups). RAID 5/6 for this volume is fine.
Given you also have SQL backups to consider - the Advanced disk pool may be a good staging location for the transaction logs before using an SLP to push them to the DataDomain for longer retention.
If you don't already have the DataDomain, then consider MSDP for backup storage. One (or two - for two copies) suitably sized RAID storage arrays would (I think) be considerably cheaper than the DD. The size of the MSDP would need to be somewhere in the vicinity of 3-400TB usable space to allow for 1 month of data retention.
Cheers