If I expand my aggregate from 14 to 26 disks is there anything special I need to run to get the data properly balanced across the disks in 7-mode or will the balancing happen in the background automatically?
How concerned about the firmware versions should I be for all of the disks in the aggregate? Is upgrading the firmware on disks truely non-disruptive, even in a very busy aggregate?
Assuming you are using the default RG size for SATA 14 (12+2) then you will be adding a nearly full raid set so you wont be too impacted by new writes to the additional 12 disks in RG2.
However I would recommend running the reallocate command against all existing volumes in the aggregate after you have added the new disks. This will optimize the performance of the volumes by restriping the data across all of the disks in the aggregate.
Do them one at a time - i'd start with a small volume out of business hours and assess the duration and performance impact while the reallocate is running.