Connecting only the RX line of a pc serial port (and GND ofcourse) should not be a problem to monitor what goes across the TX line of the PSU. Though you would need to know the actual baudrate to make sense of the data. With the wrong rate set on the pc you only get garbage or it is filtered out by the serialport hardware. Trial and miss is hardly an effective method to figure out the actual rate used. A (digital memory) oscilloscope or logic analyser would be better suited to analyse the baudrate/bit-period.
2 or more devices sending with their TX line on the same wire is going to cause disruption of each other. That is not going to work. And a short circuit could develop when 2 different levels are forced on the line.
Also are you sure that FF-cable is true RS232 signal levels (+ and - 12 volt or something), or only TTL (positive 0-5 volt).