1:
To the best of my knowledge, no-- I wish it did

2:
To the best of my knowledge, no-- I wish it did. You can however, I believe, change the 'spawn' settings for things like planet perks and artifacts.

3:
Yes-- I believe the easiest way is to simply open the properties page of the planet you don't want anything around and uncheck "Use Default Planetary Template" [or something of that sort]

4:
Weighted means it's not a guarantee of a spawn of that type-- but will spawn it more often than not. Weighted colonizeable means there's strong chance it will be a colonizeable entity of some sort, but there is a minor chance, that it may not be.

These are my understandings of these four questions-- anyone who knows better than I, feel free to correct me

Dr.Gonzo

1. Nope, that is randomly generated by the game, and is not located in the maps galaxy file. However you can switch skyboxes while playing by switching the use deep space skyboxes option on and off in the visuals options.

2. It if, but you need to mod your galaxy forge and make a very small mod for sins to handle that. I explained it here.

3. Yeah just take it off of default template in the planet options.

4. Like Dr. Ginzo said it just gives more chance of getting one thing but it could be something different. You can open the GalaxySenarioDef file to see just what the chances are.