Nothing at all stopping you. You can backstab allies, you can ambush freighters or even set up fleets right next to planets to 1 week exterminate another Good race w/o penalty to your ethics.
These things don't sound very "good" ethically. I know that GCII:ToA isn't suppose to be Star Trek, but I at least partially see the Terrain Alliance as something like The United Federation of Planets. Can you imagine The Federation (of Kirk or Picard) backstabbing allies with surprise attacks to conquer their planets? Even if they didn't have their Prime Directive, I doubt they'd do such things because they're a "good" democracy.
Even if you don't want to use the Star Trek comparision, "good" democracies on present-day Earth don't openly launch surprise attacks for territorial gains. I'm not saying that they don't have alterior motives when they fight wars for "noble" causes; but, for example, the present-day United States would never launch a surprise, un-provoked attack on an un-suspecting, innocent Canada for the purpose of expanding her northern border. If you play as a "good" Terrain Alliance, then you probably should be considered more enlightened and less beligerant than the democracies of Earth in the 21st century.
I think that maybe "good" races should only be able to declare war on other aggressors (so not other "good" races). This would also create a needed advantage for "bad" races, as they would be able to declare war for any reason; without having been provoked.
