Do not alter any files in
steam/steamapps/common/sinsofasolarempirerebellion
that's what mods are for
if you want a mod with the galaxy folder, you need to include the galaxy.manifest in the mod as well and add the map to the manifest and update the count.
Most are happy with custom maps in the Galaxy Forge window or Map Designer Window which you get to by clicking "Change Maps" which are Small -> Medium -> Large -> Map Designer -> Galaxy Forge
C:\Users\[userName]\Documents\My Games\Ironclad Games\Sins of a Solar Empire Rebellion\MapDesigner
C:\Users\[userName]\Documents\My Games\Ironclad Games\Sins of a Solar Empire Rebellion\GalaxyForge
for Win7