Well, here's my opinion, based on what I searched for over time. Its going to contradict the previous comments somewhat. LOL
There is no risk to using carbon at all in any quantity (you can't overdose you can use a spoonful or a bucketful per gallon, just need to be practical). Once its saturated, it is inert. It keeps the water nice and clean and its not too expensive, once saturated, bacterial grown on it is pretty good. Carbon is used in tertiary water treatment systems to make water drinkable, including your britta pitcher and municipal water treatment supply. There should be no negative consequences of using carbon. This part would be a fact, but I'm not providing any references to it or going to back up my claim whatsoever.
GFO I feel is optional and I use it as a boost to reduce PO4 levels when levels go high towards 1 or 2 ppm due to whatever reason, otherwise I don't use it and rely on my algae scrubber/refugium to export phosphates. GFO is more expensive for me so I balance it out and keep my PO4 levels around 0.5 ppm. If it was cheap, I'd keep levels at 0. This part is my opinion and personal preference.