In conventional aerial imaging, rule number one is: always keep the same distance to the ground to minimize scale differences. BTW: you mention a Microdrone: these can be completely programmed in 3D, so you should not have any problems following the topography. However, as Wishgranter said, PS can work with a multitude of scales. Why not fly at 460 m above the complete area and only image the lower zones afterwards? The 460 m imagery makes then your base map and is helpful in aligning the imagery of the lower zones. When you create your DSM and ortho, just disable the 460 m images of the lower zones since you have better footage of these zones.
Geert