I've done this in the past and it works quite well. I recommend that you buy a 5V, 20A power supply, crank it up to 5.3V-ish, and power directly to each Pi's he

I've done this in the past and it works quite well. I recommend that you buy a 5V, 20A power supply, crank it up to 5.3V-ish, and power directly to each Pi's header through the relay. Put a big cap over the Pi's power pins to handle transient power blips which was the most likely cause of most of my Pi lockups over time. The nice thing about relay power is that you don't need a power button. In my case I actually had a little arduino running a USB stack that could toggle GPIOs to power cycle the Pis if they wedged: https://github.com/mmastrac/pi-power-vusb https://github.com/mmastrac/pi-power-vusb (I forgot that I even added default power states and power-on sequencing there) Serial is definitely a nice to have. One USB-serial per pi, with one overall controller Pi that can aggregate it all. I would add that TFTP boot for each Pi is also really convenient. This is pretty easy to set up. Dedicate one Pi to manage the cluster power, serial and TFTP and you have a pretty robust setup.