Alliance are a serie of different mods. On the Star Wars GC, you can play the civil war Empire vs Rebel era + Black Sun and soon the Republic and CIS.
Finally the Onager and the TIE Defender Elite will be in the Galactic Empire. By the moment the Onager uses the same model than the First Order but in time, I will add probably a variant and more changes. This file from the Encyclopedia adds data inside with all the stats from the ships.