I have an 08 Tundra and I used the seven pin round thru an adapter to the flat on the trailer.
I do have an tandem axle trailer with disc brakes, but only on 1 axle. They are only the last axle and work real well. I am not sure if you would need them on both...might be personal preference on that. The brakes help to slow down the entire rig and help to keep me straight as it is, so I dont know if I would need to have brakes on all four wheels. My boat is 4100# dry, add 1500# for the trailer, 40 Gallons gas, oil, gear, etc, etc etc and I am right around 6K towed load. I am extremely happy with my trailer. It is a heavy duty trailer made with 5 inch square galvanized tubing. I will have to look at the weight rating to get the accurate number, but I know with the additional brakes the carrying capacity goes up.