You appear you may be good on payload as well as towing capacity. Look at the yellow sticker in your driver door sill. It will give you a figure of maximum weight of cargo and passengers. Let's say that figure is 1550 pounds, which is reasonable for an Expedition. If the trailer is 6600 pounds, hitch weight will be around 12% (792 pounds) and your weight distributing hitch weighs 75 pounds, you need to subtract that weight from your payload to see how much you have left. 1550 minus 792 minus 75 equals 683 pounds remaining for luggage, bicycles, coolers, firewood, and people. Bottom line, read your door sticker and do your own math. You vehicle's payload capacity may vary significantly based upon the options ordered with the vehicle.
2014 F150 SCrew 4x4 Max Tow Heavy Duty Payload 3.5 Ecoboost 6.5' bed Ingot Metallic Silver
2015 Rockwood Roo 21SS