The AQMF minis and figures are effectively about 1/100. Airplanes in "true scale" will appear to be too large when close to the other minis. Because the planes will use some single point (agreed to by the players) as the measurement point, any size airplanes can be used. The smaller scale planes visually look good.
There was no backdoor at the Alamo. Planet Earth doesn't have one either. Fight and win or DIE!
Use of the balloons and early war planes work. All should get shoot down easily. For the balloons maybe give heavy artillery TWO turns of pre-bomberdment? Crew auto bails when Martians can reach them with any weapon. Maybe allow one unit of tanks to hide like infantry.
Our group had this discussion a while back. We decided that since aircraft will be high in the sky they will look smaller, so the smaller scale models will work fine. If it is landed it is a different story.