I think that the costume looks sweet so far. I have worked with cardboard before and it can be annoying sometimes. Are you going to make boot-type things for the legs? They kind of taper off into round feet so thats just what i figured.
And yes, your info on the grunts is correct, they are 5 feet tall, as are the jackals, the elites are 8', and the hunters are 12' tall when they stand up, and only 8' in their battle possitions. The reason everything looks small in the Haloverse is because the master chief is so epicly huge.
keep up the good work!