is there a way to garranty that a planet will be militia controlled?
Yes, but you have to edit the file in Notepad, as GalaxyForge doesn't support a "RandomMilitia" owner option, even though that is a valid option.
I don't want to plug my stuff, but my tutorial map makes extensive use of predefined militia and militia owned planets if you want a reference (again open in notepad, not galaxy forge).