At 6 days old and since you saw problems early on, I doubt if it is coccidiosis. I would do the NutriDrench, but they won’t probably have any harmful effects if you continue the Corid as well. But they may also be getting too warm. With chicks I only make the heat about 90 degrees the first week...
Sorry for your loss. Hopefully others that have Guinea fowl experience will chime in, but with 3 days in shipping, my guess would be that they suffered from shipping stress and possibly overheating or rough handling. In that case offering fluids with electrolytes, poultry NutriDrench a couple of...