Would it even be possible to make a mod that sets the game in the Halo universe? I think it would be pretty cool to watch space battles (think of the Halo books) and travel to all the diferent planets.
...what story? You don't need to put an entire galaxy of planets on one map... You can create scenarios using specific star systems, like the warhammer 40k folk do.
You want to alter ship models to match Halo designs? Go for it. The only problem is that there's no ground warfare or ship-to-ship boarding battles in the Sins engine, so you lose the majority of the Halo universe. Not worth it, in my opinion, unless you're a Halo junkie with great digital art ability. Even then, waste of your talents.
Pillar of Autumn had a triple charged Guass/rail gun, capable of firing three successive shots which was enough to cripple and if lucky destroy a Covenant cruiser. But the the Pillar was a Halcyon *Advent? hehe) cruiser known for its resillience so it kinda was better than other ships of the same size. However it was through sheer luck and tactics that won it for the UNFC (no matter how few there were).... Covenant were just better.