It's my understanding that all the non-BEC Castle ESCs must be fed 5 volts to power their logic.So you would need to connect all three wires from all the ESCs. You probably need to use only one of the two BEC ESCs you have though. So one with a BEC and the remaining five without BECs.
I'm not familiar with the specific layout of the Y6 PDB. If it has three points of connection for each ESC then that's fine. Plug them all in. What you don't want to do is the standard ESC practice of only connecting the +5 volt lead (center wire) on the main BEC ESC and not connecting the center wires of the remaining ESCs. The non-BEC ESCs need a +5 volt supply to operate.