Basically, it is unnecessary to Normalise ALL tracks before or during the mix operations. Try to get the gain per track around the -6 to -9 decibles to start with by using the Gain plugin. You do need to have headroom on the gain sliders. When you have mixed the tracks, you can used the Merge to Stereo Wave ensuring that the output is not being clipped (flat heads to parts of the track). THEN apply Normalisation to the final wave to maximise the sound level with a headroom of 2 decibles allowed for further adjustment.