Oh, even in a properly-stocked pond, they'll still knock each other around. Sharkie's barreled her fry out of the way before. It's less that they're fighting each other and more that they're all very focused on one thing, the food. Carp are the hungriest things I've probably ever encountered, to be honest. But they're dumb wet puppies and I love them dearly.
That being said, despite the ones in my pond all knowing that the top of the waterfall contains food, only Peekablue jumps it on any regular basis. There's enough food in plants and other stuff to keep the other ones happy without ever wanting to jump the falls as long as they get regular meals. Peekablue has also been known to jump over the other koi to get at food, though, so I think that's just his thing.