In multiplayer/skirmish (or all) scripts in OP/EAW (and most), the first 6 starting positions G > L are the positions of the players involved; (...) So changing G H I J K L will change where players 1 2 3 4 5 6 show up.
Yes, that will be the case in most of the maps.
In the text file, that comes in the ZIP file, we can see which are the default symbols that can be used in the maps.
But, as any symbol can be overwritten, we should be careful before assume anything.
It is always a good idea to study the maps, of the script we want to edit, before doing any change. Because the developer of that script could have overwritten something and changed its meaning...
If memory serves me correctly (and that is by no means a statement of faith), placing large objects such as planets/stars in adjacent spots will crash the script, so try to keep a few "." between these. Asteroids and dust clouds shouldn't be an issue
The scripts apparently doesn't crash. But it is always a good idea to give some space for larger objects so they won't overlap.
Bellow is a screenshot of something i tested before writing the mission editor
My ship is somewhere in the middle... but the IA didn't survive... ^^