Okay, the level where you flashback to Buck, and you have to go and beat the Covenant to Dare, there is a part in the level where dead Brutes AND Elites can be found. When seeing this scene, you can tell that the Brutes and Elites killed each other.
Now, the part where I think the dev's messed up is the timing. The flashback takes place thirty minutes (or an hour, don't remember) after Buck lands. The Great Schism however, takes place about two weeks later after Regret does a SlipSpace jump from New Mombasa. The game takes place just a few hours after the jump, while The Great Schism takes place weeks later.
So how is it that The Great Schism started hours after the ODST's drop in, when in Halo 2 it takes place a few weeks after Regret leaves Earth.
You can tell that the Elites and Brutes killed each other by what Buck and Dare says.
Buck: "What's with all the dead Elites around here, was there a family feud?" (something like that)
Dare: "That's classified."
Please, feel free to share what you think.
Read my FF! Human-Covenant War: Without The Great Schism. It's an FF based upon how the war would have ended up if there was no Great Schism. There are a lot of twists and there is a lot of action!