These two Pundamilia sp. males have set up spawning pits about a foot apart in my mixed tank. They fight constantly but without injury.
Here's a video of them sparring. You might want to click 'high quality' below the right side of the image, since air bubbles in the water make the regular quality image look very grainy.
http://www.youtube.com/watch?v=f0y-ze-usvY