OK, time to backup a little here.
Installing 'some' of the tenant feeders from the load side of their respective main CB should not be an issue. You would be making connections on the load side of the CB within the gear. Think similar to installing a branch circuit into an existing panel.
Yes, caution must be paramount when working within an energized panel, even though you are connecting to the load side of a feeder CB.
My opinion, and I believe Reno is going the same direction is that the design MAY think about a 'MAIN' for the possibility of future needs or expansion.
It is my belief that you may be correct re connecting to load side since I expect that an expensive and large Switchboard should have the real estate and provision for a trained pro doing so with caution of course. I would have all 6 breakers powered down for even more caution.
As you suggest I would like to install a main ahead of the 6 pack but it is a cost driver and is not mandated by NEC rules so customer will question. Again he (like so many) thinks the future will see no change to his current plan.
