[Edi]

Border mountains are mountains for the purpose of magic site determination and resources, perhaps also population. They just don't hinder movement.

If you go in and edit the map file after generation, you can mix proper mountains and border mountains as much as you like. What you do want to do beforehand, though, is copy the #defaultmapzoom command value out of the file, because the map editor currently overwrites it on save. That is fixed for the next version, but 3.06 screws it up and map icons will look wrong without the correct defaultzoom.