Decoupling for C-IV GX

Both are possible. Just be careful about the placement of your decoupling capacitors to reduce the high frequency supply and return current loops as much as possible. Generally I'm using a dedicated power plane for VCCINT, two other power planes with islands for the different VCCIO, VCCD_PLL and VCCA, and at least 3 ground planes. But this depends highly on your board routing and how many voltages you need to provide to the FPGA.