BRS has a vid on their website about using their reactors, they give you a recommended formula for mixing gfo and carbon. Gfo will fuse together and lessen the efficiency by limiting the surface area, that's the main reason to tumble the gfo. But yes you can mix them, mixing gfo with carbon will separate the gfo so it won't fuse together, go check it out. There are some drawbacks as gfo tends to last longer than carbon, but if you are only running one reactor, it's a good option.