Ok... on the left you have the model from Mass Effect 3 and on the right the one I made
I find a way to separate normal textures into two part: all the finest details (skin pores etc...) and all the larger bump (that are specific to that model such as scars etc...). The potential of this method is that, recombining the two textures, you obtain exactly the input texture. So I take the normal with Cullen's skin pores and I overlap it to Kaidan's default normals. I think I've done a good work! The base mesh on the right is Cullen head that I turned into Kaidan one, so it's weighted with DAI default bones. I need to fix the hair a little more. I also worked on the diffuse and specular textures, but I think that this reder emphasises the work on the mesh (I recommend you to see this in full resolution)