Hey Osias,
As far as I know this tutorial series from Epic Games explains best practice with defining different surfaces within the same material.
Well worth a watch: https://www.youtube.com/watch?v=lngF4VVNER4&index=79&list=PLZlv_N0_O1gaCL2XjKluO7N2Pmmw9pvhE
Hope that helps! 
Should be simple,
Metallic texture mask - Black where there is wood, white where there is metal
Roughness texture mask - lighter greys where there is wood for rougher surface, darker greys or black where there is metal(unless your going for a really rough metal look)
Specular should be left as a single scalar value set to whatever the reflectance of wood is. (I don’t think this value has any effect on the metallic areas)
If your getting errors in your nodes, check the console to see what the error is. You might be trying to blend two different types of floats values(float2, float3)
I don’t know why but now I am not getting any errors when using the lerp and plugin everything up. I am also getting the expected results.
I guess its just I am finally starting to understand the system, well sorta and at least I think so lol Finger Crossed.
Thanks for the help everyone