I think the Flood is the most possible answer.
-Being a Brute or a Grunt contradicts everything on Xbox Live. For example, Grunts are way to short, and would make it strange for them to jump the same distances as the Master Chief or the Elites. Also, Brutes or Grunts do not have shields, so the Plasma pistol would be useless.
I vote Flood, because they have shields and can weild any weapon.