You can't really change the symbology of a layer in arcpy with the level of customization you have when just right clicking a layer and using the symbology menu.  However, you can make a "template" layer that contains the symbology you want and then use `ApplySymbologyFromLayer_management (in_layer, in_symbology_layer)`.  This will allow you to apply the symbology for any layer with that of the "template". Then you can use: 

    yourLayer = arcpy.mapping.Layer(r'path to your layer') 
    yourLayer.transparency = 50 #as a percentage