I used broken up salt licks until the pieces get too small -- say smaller than my fist. I definitely wouldn't put golf-ball size rocks in the feed dish. Horses SHOULDN'T eat rocks, but they shouldn't choke either. You can also put a bit of vegetable oil on the feed to make it more slippery. This helped my last choker a lot until she developed "normal" neck muscles. Really. I'm not kidding.
I'm not a fan of mushy food unless there are teeth issues or it's something like beet pulp. They can still scarf it down and choke anyway IMO. Go with the rocks and like suggested also -- the flatter feeding bucket so things spread out more.