I will have 2 mcp-655's ... and it's rather loud, i'll test it, whats a few bucks. this looks cool too ... plug in some fans!
And don't run your pumps in electrical series. 6V might be too low and could stall your pumps.
Anyways you don't need any extra cables at all. Just switch the two pins on the molex connector.
Right now your pumps have pins in one of the grounds and the 12V line. Take the wire going to one of the grounds and hook it into the 5V line. The electical potential between these two lines is 7V. Basically this is what that 'adapter' is doing.