The only thing I did to the limited edition chap was stick some green stuff to his face it will come off easy enough if I need it to =D and the reason I have an urban squad is COD had just come out and I had a 7th squad laying around but still wanted them to be linked to the army. But when the new codex comes out I will do any new units in this scheme as it is so much easer to do compared to my original army
Feel free to use any ideas you see
As for how I did it well they where done a long time ago but if I remember correctly I only used green stuff and the standard tool you get from GW.
First I started of with the mask. This is relatively easy just make sure you have left the green stuff for 20 minutes so it is not so sticky and use water/oil/spit to stop it from sticking to the tool so much. Put a bit of green stuff over the mouth and then using the small flat bit of the tool shape it and square it off. I decided early on that I would have that bit running up the middle of the head so it covers there nose(at least I think it is there nose). Then use a knife to gently cut/scrap away any excess and to give it its shape. Then with a lubricated sculpting tool (or any flat metal will do) smooth it over. Leave that to dry then do the same with the eyes but with a lot less green stuff
A good idea is to get an old head..... non Tau cos they are hard to come by I find and practice on it.
The gun straps come from the kroot and the cut down pulse rife is just a carbine with the launcher cut off.
Also the left shoulder pad is just some shaped green stuff
The Tau heads where relatively easy to do especially compared to the Eldar warlocks I did on jetbikes there helmets where a lot more work but I used the storm guardian head as a basis.