It is a dual-controller system, right?
Are you adding all 24 new disks to one of the controllers only? If you don't tweak anything, then you will end up wit a pretty neat setup IMHO:
- RG0 - 16 disks (no change)
- RG1 - 16 disks (6 old + 10 new)
- RG2 - 14 disks (all new)
- 2 hot-spares
Nothing wrong about this in my view.