The Warthog's looking awesome, I'm really excited.
Jumping back to the topic of who-can-play-Grunts, have you considered having people crouched and molding the body to accomodate them? Grunts have an awkward, lopey run as it is.
Here's a crude Flash drawing of what I mean.
The green being the general shape of a Grunt,
The red being the crouched, incredibly uncomfortable person inside,
The dark blue being the supports or extra parts to enforce the costume.
They'll need to be leaned forward a bit more so as to not lose balance when they wobble along. Haha.
Or maybe this is just another of my crazy ideas.
But that was the first thing that popped in my head.