Just some general info on aircraft load plans based on the Air Force Load Planner's Course.
The two most important considerations are the maximum safe lift capacity of the aircraft and maintaining the aircraft's center of gravity within an area that will not cause the aircraft to become unstable in flight.
The first one is a 'no-brainer' but the second one may require some explanation. The aircraft has a fairly well known center of gravity when unloaded. Adjustments to trim, etc. will keep the aircraft stable in flight with little effort from the pilot. Adding large amounts of weight to the aircraft by loading it with troops or vehicles can change the center of gravity of the aircraft beyond the range where 'trimming' the aircraft is practical. This makes the aircraft unstable in flight and requires a lot of extra effort on the part of the flight crew to keep it in the air and pointed in the right direction.
For instance, loading a few PzKpfw IIIs in the nose and 'pax' (passengers) in the rear, will probably make the aircraft nose heavy. (Pax with equipment are still estimated at 200 pounds each.) Switch the pax and the AFVs and the aircraft becomes tail heavy.
Transverse movement of the CG is also a problem. If you load motorcycles or guns one one side and pax on the other, the aircraft will have a tendency to turn a lot faster in one direction than the other, even with counter-rotating propellors.
There is a spherical area within the aircraft where, if the Center of Gravity is within this sphere, the aircraft is considered to be flyable. The load planner's job is to get as much cargo as possible, including pax, loaded onto the aircraft while still mainting the CG in the safe area. The load planner disregards the distribution of fuel in the aircraft fuel tanks, a significant part of the load, and the flight crew is responsible for making sure that fuel distribution does not move the CG out of the flyable range.
For each aircraft and load, there are usually several possibilities for the load plan. If this is the case, the load planner has to take into consideration what part of the cargo needs to unload, or be unloaded first. If the mix is pax and AFVs, the pax will probably be arranged so that they can unload first, because the AFVs need to be tied down and chocked, so they take longer to unload.
I almost forgot that this tying down is the third factor. Any loose cargo that shifts during flight changes the position of the CG. If the crew can't maintain stability in flight, there is a good chance that the cargo will shift to a point where it will cause the aircraft to crash almost immediately.