Does the Galaxy forge have the DLC's new planets?
I think so, if not it is possible to add them to it manually without too much grief.
how do i set up a designated starting point for player?
Each planet has an owner and homeworld property.
Am I able to determine the amount of resources and artifacts the planet has? and if so how?
When you edit the players you can set their starting resources. You can change the artifact density from the main menu (when you don't have an object selected), or you can manually add one to a planet by adding a planet group to the planet, adding the artifact you want, then using the settings "Always" and "NoOwner" as parameters.
how do I add file to game?
Put it in your C:\Users\User\Documents\My Games\Ironclad Games\Sins of a Solar Empire Rebellion\GalaxyForge folder.