Unfortunately, the only camera apps that can currently be set to default on a Lumia are made by Nokia & Microsoft.
It's a shame because there are some cool camera apps like ProShot and Camera360 that I think would be fun to have at the click of the camera button. But I believe that there are certain restrictions in place to protect the overall stability of the OS. I might suggest pinning the tile for your camera app of choice to the tile closest to the camera button. That might make things easier for you.