If I combine photos from a 20mp P4P with photos from a 12mp Mavic in a map, will the visual quality of the map suffer due to the addition of “lower quality” 12mp photos?
I do not anticipate your map’s quality suffer due to the addition of the lower resolution DJI Mavic images as long as the images from each camera are correctly aligned with one another.
The best way to ensure that the images from your two cameras are aligned correctly with one another is to consider the images’ ground sampling distance.
More information about the GSD here.
You can get the same GSD when using a 20 MP or a 12 MP digital camera. In that case, there is no problem in using both cameras together and you will not lose any quality.
Even if the GSD is slightly different, you can still use both cameras. You can find more information here.