you can run carbon and gfo in the same media reactor, you should only run gfo if you need to lower your phosphates and that can only be answered after the tank is up and running and the water tested. I view carbon as an an insurance policy for when (not if) a coral gets mad or dies and starts a toxin war or something else gets in the water and needs to be removed quickly. While carbon isn't essential, I would not have a tank without it running for peace of mind. It also helps the water stay clear - a plus for ensuring proper lighting reaching corals
you can start with carbon in the reactor and then, if still needed, add gfo. easier to do than trying find space for a reactor after everything has been set up.