You know, I saw this thread title, and I turned to my girlfriend and said to her "I just saw this thread about a Halo Mod. I imagine it's going to be one of two things: Master Chief in Sonic Robo Battle 2, which is going to be lame as hell; or it's going to be a Sonic mod using the Halo engine, which could potentially be as awesome as hell."
Unfortunately, it's the former. The Grunt and Elite both look pretty decent, but the Spartan looks really horrid, I highly recommend trying to make it more anatomically correct.
Kudos to you for going through with your project though. :)