1. Yes, Gas Giants can be made colonizable like other planets, by editing the PlanetGasGiant.entity file. Change values:
isColonizable FALSE
to
isColonizable TRUE
if it doesn't work, try changing the line
planetTypeForResearch "Uncolonizable"
to
planetTypeForResearch "Terran"
There is also a way to make it "legit" by creating a new research topic for Gas Giant colonization, but the above should work.
2. It is possible to have a planet which has virtually no ground presence benefits, only orbital building capacity. In fact, looking at the PlanetGasGiant.entity file, you will notice all of its upgrade stages have their values set to zero. Upgrading costs nothing and gives nothing. In addition, the planet's health is set to 1, so a sneeze would kill it. In order to make it viable, you would need to edit all those values to what you want.
3. Not really sure, but probably yes.
4. See here, I explained the process a bit (3rd post down):
WWW LinkHope this helps somewhat!
