Just append one of the materials and you are good!

Yes, but be aware that the way Blender handles it, it will propagate the changes to every material using the node. To avoid this, be sure to hit the little number between the shield icon and the node name, to create a new copy of it

Yes, it is the way it was conceived for, in the first place! Be aware though, that it follows a Specular-Glossiness setup, so if you have a standard roughness map the channels are inverted. Use a color ramp to maximize customization, and use the sliders to increase the overall effect