We had an event a couple of months ago where the Rebels were attacking a space station. we had to take out a shield generator on a planet and a drone replicator in an asteroid field and then attack the station itself. Mainly it was up to the organizer/planner to try and set things up on a balanced fashion, but I was still hard to do without play testing. Needless to say the rebels were at quite a disadvantage.

Are there any official rules for building balanced scenarios for attacking space stations?

Nothing official, but the closest thing that's playable would be this:

http://dockingbay416.com/campaign/

Heroes of the Aturi Cluster is a superb work that has lots of options for running a campaign or even one-off scenarios.