Relay: Minimum switching voltage
You are confusing the relay coil (control) ratings with the relay switch contact ratings.

The voltage across closed relay contacts should be approximately zero.

At extremely low voltages, there may not be enough energy in the switched circuit to break down any oxidation or other insulating layer on the contacts - this voltage will depend on the contact materials.  The relays you are looking at will successfully switch voltages as low as 100 uV, and the contact resistance will be under 50 milliOhms

The operate voltage is the voltage you apply to the relay coil to operate the relay.

For the relay you linked to, the Rated Voltage is the voltage that should be applied to the coil to operate the relay.  The Operate Voltage is the minimum coil voltage at which the relay is guaranteed to operate.  The release Voltage is the minimum  voltage at which at which an operated relay is guaranteed to remain in the operated position.

A DPDT relay can be used in place of an DPST (the double-throw (form C) contacts are much more common than single-throw).  Just ignore the unnecessary contacts.