Hello everyone, I am a beginner in drones. Currently, my mentor wants to build a hexacopter with a full load capacity of 30kg to 35kg, and the desired payload is 5kg; of course, the larger the payload, the better. The flight controller I have chosen is Pixhawk, and the frame has a wheelbase of 1600mm. I am unsure how to choose the flight control firmware, and for the motors, I am selecting them based on a thrust of 5kg per motor, then choosing the appropriate ESC. Although this may seem a bit silly, I really don’t know how to proceed and sincerely hope to receive advice from all of you.
Get a subscription to ecalc.ch and it will help you with some of your estimations and component selections. 30kg of thrust (5kg x 6) will not be enough to control a 30-35 kg drone.
Since you’re posting on the Ardupilot forum, the only answer to the firmware is Arducopter. 
Look up the Ardupilot Methodic Configurator. It will help you set up your drone once you have the parts. It might seem like a lot at first but it will save you a lot of time and stress in the end.