Powering up grid of 20 Raspberry Pi 3
1/ The SP-240-5 does only 45 Amps so I would avoid that one.
 Then yes, you can wire them up. Your approach of sticking to the 50Amps although it 'looks like' you get away with less is correct. The load also depends greatly on what you are running on the ARM cores so don't assume that as long as you don't use USB/WiFi/Bluetooth, your OK. I assume you are not connecting other stuff like cameras HATs etc. 

2/ I would add a fuse per Pi. Yes, the Pi has an internal fuse but with 50/60A I personally would like to have a small glass cylinder with a thin wire in front of each PI.

You can only switch them all on or off. Maybe add an on/off switch in front of each.