The pellets can only reduce PO4 so long as they can reduce NO3. Once NO3 is 0, they cannot reduce PO4 any further. Thus, there is a case to be made to run both GFO and pellets.
Carbon should be run on a reef tank regardless. Not a good idea at all not to have carbon in there as there are too many allelopaths released by corals, (softies and leathers are especially bad for this), plus it polishes up the water clarity. Here's a good writeup on the topic:
http://joejaworski.wordpress.com/200...k-need-carbon/