There's a couple of reasons here, if you are running a reactor. If you run both GFO and GAC in the same reactor, the GAC will need to be replaced more frequently than the GFO, so you would never get the full use out of the high capacity(and its added expense), or the GAC would become useless long before the GFO. The same philosophy would hold true if you prefer to change out your GAC and GFO together on a regular basis, as part of a maintenance routine. So, to get the most use out of the high capacity GFO, you should be monitoring your PO4 and only replace it once it begins to elevate. Also, I think you'll find that the pellets would be recommended in using a passive method, such as filling a sock, and placing it in an area of high water flow.
If you are running them in a dual reactor, you can still change them out separately, so it would a be your choice as far as which GFO. As far as the flow is concerned, you will need to adjust the flow to keep the GFO (whichever one you prefer) from clumping, usually a slow roll seen at the top of the media. Do you already own this type of reactor?
nope, I'm piecing everything together right now for a build in the fall once I move into my new place.