If you're doing 500g, the posters before me are correct, stick with goldfish as the pond is too small for koi. You can do sarassa, shubunkin and comets. All available at your local pet stores like Petsmart and the like.
No shelves, as this will only serve as an easy spot for your goldfish to get snatched by a predator. You are giving them a good spot to stand and go fishing. Also, no rocks in the pond. It makes it impossible to clean, and food, poop and other stuff just gets under them and you can't get it out--then it pollutes your water.
before you dig any further, what are you thinking about for filtration. I would take a deep read on our pond construction forum, as you can learn a lot there.