To go along with my armor I made a couple of the parasites. I made the original out of clay, but it was so tiny and delicate that I would have been very difficult to cast. What I ended up doing was getting brush on latex, armature wire, masking tape, and newspaper. The main body is built up with newspaper and masking tape. The wire is inserted into the body portion and built up with more masking tape. Then I just went crazy with the latex until it was thick enough to pose and hold up. Airbrushing some discolored skintones makes it a bit more life-like. To finish it off I just went in and did detail work with the veins. The first one turned out well, so I made the second along the same lines. No drip latex works best, or else you have a lot of sanding.
Some people might like that melting infected look though.
As far as elites, brutes, and grunts. I'd start with the grunt because it's the most geometrical and easy to construct.
The elite, I'd probably go with some sort of skin suit and sew pieces of fabric onto it. Some sort of blue/teal/cyan/or cobalt colored vinyl would work