If i remember correctly ice shedding is done at 80% N1 on the 737.
Re the TOGA issue, in the airbus world that would be indeed full take of power, reduced thrust is set in the FLEX/MCT detent. In the boeing world it is as described by wiggy, simply the system to set the calculated take off thrust which can be considerably less than full. The 737 allows "double thrust reduction" by both using derated thrust levels and the assumed temperature method.