The key to removing galvanized pipes is to have two really good pipe wrenches. Use one to lock on to the piece that should not move so as not to twist the rest of the installed system. Use the other one to loosen the length of pipe to be removed.
Start by using penetrating oil on all the threads. It's a good idea to have two pipe wrenches for unions. Sometimes the oil won't work, so it's helpful to heat the fittings with a propane torch. Last, you can cut the pipes being sure to use a metal cutting blade.