Well...I went to Hobby Lobby, they didn't have anything but 12mm diameter magnets so I got a pack.
DON'T use 12mm!!! They work so good that cars can't roll over them without hesitating whenever the axle passes over them. I didn't notice any affect on the engine and they most definitely opened the Kadees, but with the hesitation of each car the couplers re-coupled once the car overcame the force of the magnets.
I may still try the 8mm or less diameter. I also tried double-faced tape (didn't work) and super glue (worked) to hold them in place.
Question, could these magnets interfere with the electronics in the engines? All that battery-powered stuff I have got me thinking!