If a material is 100% reflective, the diffuse doesn't matter right?
No. What's stored in the diffuse texture always matters, it just means different things in different cases. In the setup you're trying to use, if a material is 100% metal, the value in the diffuse texture is re-purposed to be the F0 value. Otherwise, for non-metal (dielectric) materials you hard-code F0 to ~0.03-~0.04 for most materials (see Hodgman's comment), and the value in the diffuse texture is the diffuse color you're used to. I haven't used Marmoset, but from seeing what a few other places are doing, the 0 to 1 mapping for F0 will probably do something like change the value from ~0.01 to ~0.08.
I think what you're referring to as reflectiveness is what most people refer to as roughness (or shininess). This is stored in its own texture and is used alongside the others to control how shiny the surface is - this is what controls the specular highlights for direct lighting, and the mip selection for indirect lighting. It is used as part of a larger equation to control surface appearance.
Most approaches use the following (or close variations) as the starting point of textures using physically based rendering.
Diffuse (or Base Color): for dielectrics, exactly what's in the texture. for metals, the f0 value
Metal: 0 - dielectric. 1 - metal.
Roughness: how rough the microsurfaces along the geometry are (alternatively, this can be Shininess, which is just 1.0 - roughness)
Normal: plain old normal map
There are a few variations to the above that different engines use, but that should give you the gist of where values are coming from.
As far as finding actual code used for PBR (btw, you need to read this stuff - start with the Physics of Math and Shading presentation and go from there):
http://blog.selfshadow.com/publications/s2013-shading-course/
http://blog.selfshadow.com/2014/08/12/physically-based-shading-at-siggraph-2014/
http://blog.selfshadow.com/publications/s2015-shading-course/