As for the size of the rock, my opinion is that what I call "egg rock"
would be most attractive. Those are about the size of a person's fist.
You don't mention whether the waterfall is made of boulders or how
much surface area it represents. You might post pics if you can, and
you surely will get a lot of suggestions that way. Landscape fabric
over the grid is also a good idea.
If you find a koi club, be sure to let them know why you are getting
rid of the fish. Find out from the people you bought from, if you can
cotact them, to see whether the fish are "pet quality" or something
more, and also solicit an opinion from the folks on koiphen, as
disinterested but knowledgeable parties. I wouldn't expect to make a
lot of money, maybe not even enough to fund your water feature idea,
but anything helps. Spring is a good time to be sellling them.