Hey Kevin,
I have several C6100s (XS23-TY3, latest BIOS and BMC fw) in a nearby colo but I have all nodes set to power ON upon power being restored. I haven't tested any other BIOS power-state options. I have run into a weird bug though... I had all nodes OFF (manually turned off using the rear power buttons) and then after 7-8 days they all turned back on automatically (no power interruption occurred). This happened a second time 7-8 days later after I turned all nodes off again. I suspect there is some kind of counter/timer overflow bug in the BMC that causes it to crash every 7-8 days, and when the BMC comes back up it automatically starts all nodes back up too. I can't leave the nodes off for another week to test if other BIOS-power states can be used to avoid that bug.
Maybe you could buy (or make) two filler panels, and only have two nodes installed initially? I never tried running a chassis with anything other than 4 nodes, but I don't see why it wouldn't work since nodes can be hotswapped. It's possible (but I doubt it) that node 4 may need to be present at chassis power-up; I think it's the 'master' node (the one that allows you to configure chassis-wide power budget settings in the BIOS)?
I've never used a DRAC card, only Supermicro IPMI and C6100/2100 BMC. The main BMC features (KVM and remote media mounting) work, they are just klunky and Java-based. It's very similar to Supermicros IPMI interface. I've never had any real problems with the C6100 BMC other than it being a bit slow at times. A manual BMC reset (or even just waiting a few minutes) fixes that.