Great job on your build, Roy! Looks very nice!
To respond to some of your comments:
The wire that Frozone was referring to in regards to not being gauge specific are the wires that go between the PID and the SSR (terminals 7 & 8 on the PID). This uses 24V and only 30mA of current and he is correct. I think what you are referring to is the output of the SSR that ties into the heater. This side of the SSR would need to handle the current of the heater. If you want to size it for under 1200W, go with 18ga or larger. If you want to size it for the capacity of the SSR, you would need to use 10ga.
The mounting of the probe in the fashion you show will not affect your temperature readings. Just be sure that the mount is not going to interfere with adding material, fluxing or cleaning. If you mount the TC < 1/2" of the side of the pot, temp will show a few degrees higher than actual because of its proximity to the heating coil.
Finally, the temp difference could be numerous things. I would tend to believe the TC is not at fault here though since the dial thermometers are not as accurate and can get knocked out of calibration.
Suggestion to use to check which one is off: Lead melts at 621°F. So set the PID at 675°F for a test. At this temperature, your dial thermometer should read below 621°F. If lead has melted, you know that the TC is accurate. If no lead has melted, you know that the dial thermometer is accurate.
If by some very strange chance the TC is inaccurate, you can adjust its offset in the settings.