I'm talking about the ingame editor, not the standalone Galaxy Forge.
I was thinking maybe there is a way of altering the way the ingame editor works, perhaps by modifying some files that it uses?

Thanks for the tip. I was able to remove the militias by replacing

planetItemsTemplate
templateName "Template:DefaultStart_Terran"
subTemplates 2
template "Template:DefaultHomePlanetSetup"
template "Template:LocalMilitia_Terran"
groups 0

with

planetItemsTemplate
templateName "Template:DefaultStart_Terran"
subTemplates 1
template "Template:DefaultHomePlanetSetup"
groups 0


I couldn't find a way to only make the player start with one planet though. And that still wouldn't be enough, I don't want the home planets to be as far out from the sun that they are by default either.
Basically I want an easy way to create two random solar systems, each with one player in it but with lots of neutral AI's. The ingame editor lets me do all this but puts the starting planets too far away from the sun(s) for my liking.

This bothers me too about the in-game edit, but I have never found a way to stop it from doing it. Even with all the distance sliders at minimum, it still separates those few starting planets considerably.

What you may be able to do, though, is to create the map with the in-game generator, then open it in Galaxy Forge and move those planets closer. You would have to assign starting positions to the players then for it to work properly (the in-game generator does not create starting positions).

This is also the reason why Galaxy Forge maps from the user path Galaxy folder auto-end when started. They have start positions, but the game gets confused because it only wants maps without them from that folder