Build your high poly model, and bake an "ambient occlusion" map as well as a normal map for details that will be lost when you convert to low poly. Bake your diffuse at this time as well too.
As long as your UVWs are projection mapped properly, or your UVWs are matched manually (however your personally do them) any texture can be used for a normal, diffuse, or specular map. In fact, baking detail to materials from a high poly model to a low poly is how 'good' texture work is done.