The nature of rice cakes means that a stickier rice is preferred. Short grain rices are stickier and softer. Obviously, "sticky rice" used in suchi is very, well... sticky. Medium grains, like Valencia or Arborio are creamier. If you must use long grain, use more water than normal, and cook until all the water is absorbed. It will be softer than normal, but soft is really sort of mealy and isn't really "sticky," which is what you would like to hold the cake together. And don't rinse before cooking. You want all the starch.
Follow your rice cooker directions, which I suspect will call for a little more water than rice. (I don't use a rice cooker.) And the advise about not washing may not be valid with all Asian rices. Some imports have had talcs and/or starches added, and I want to wash those and trust to the starchy nature of those rices on their own. The amounts of water recommended assume fresher rice. Older rice needs more water.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen