Stick with aluminum for intercooler tubings. Buy couple of U bends and start cutting and connect with silicon couplers. You want aluminum for the heat dissipations. Using mild steels or especially stainless steel will soak in heat.
As for the sizes of the intercooler pipings. Don't order the sizes until you have the intercooler and turbo in your hands. Because the inlet and outlet of the intercooler will dictate the sizes of the pipes. Majority of the times, they are 2.5 but sometimes 2 or 3 inches depending on how large the intercooler is. Then also the sizes of the compressor housing of the turbo will come into play.
For the turbo manifolds. Some people will use mild steels to help keep the costs down but those will crack in about 6-8 months. Or those Ebay stainless manifold from China, they will crack just as fast too. If you want the best, some company uses schedule 40 stainless.
http://www.amazon.com/Dirty-Dingo-Motorsports-Turb...
Here is one I really like and will probably use when I am done with my LS swap. But you have to be fluent in turbo for this manifold.
http://www.ebay.com/itm/271697172489...%3AMEBIDX%3AIT