Assuming this is for a 29gal volume:
GAC: about 1/2c
GFO: about 2 TBLS total, however I would add 1 TBLS wait 2wks then add the 2nd TBLS on top. Replace 4-6 wks unless if you are closely monitoring Phosphates, then replace when the output from the reactor is 0.02 - 0.03 mg/L.
I rinse GAC and GFO in RO/DI before use to cut down on the dust. I also run the Phosoban reactor output into a 100 micron filter sock.
The only problem with mixing them is GFO requires slower flow 80-100 gph, while GAC is more optimal at 100-140 gph. Other than the flow and causing the breakdown, there is no problem with the two in the same reactor.