Are the power cords interchangeable from unit to unit?

No. Only the FL-8s and FL-18s are interchangeable. The FL-12 and FL-20's use the same power cord, but will NOT work with the FL-8s or the FL-18s.

Will my old transducer from my FL-8 or FL-18 work with my FL-12 or FL-20?

Yes, all transducers are fully interchangeable between the FL-8s, FL-18, FL-12 and FL-20's.

What improvements does the FL-20 have over the FL-18?

The FL-20 has a flat screen that delivers a larger display that won't trap in rain or snow and increases the viewing angle by 50%. It has two auto zoom ranges that split the screen to let you focus on the bottom 6 or 12 feet of the water column. It also has a night mode that allows you to cut the output of the LED display by 50% to avoid getting night blindness. The FL-20 also has a built-in low battery indicator. But note, teh FL20 DOES NOT have a BL or Bottom Lock option for open water use.

Does the FL-20 have Interference Rejection like the FL-18?

Yes, it's virtually identical. There are ten IR settings controlled by pressing in on the Gain control.