You'll end up with 2 x private groups with different drive counts, and likely inconsistent performance across the pool.  For consistency, you really should be adding to the pool in drive count increments the same as are already in it.
Hello,  On a VNX5200,  version 05.33.000.5.081.  I'd like to expand my storage pool "Capacity" part by adding NL SAS drives. Actually, I have 8 drives in this one, with RAID6 (6+2).  The wizard recommends me to add 8 more disks, but i'd like to add only 4 of them. I have the possibility to do that in the drop-down list.  What's the consequences of adding less disks than recommended?  Thanks!
VNX5200 : Add 4 disks to storage pool instead of 8 as recommended