What will be the coordination number in Fe(0)?
This is another data point for my constant mantra that using hybridisation to explain coordination chemistry is not helpful in the slightest. And even ‘worse’: you are trying to invoke some strong meaning of the 18-electron guideline even if not explicitly. The 18-electron guideline is a guideline not a rule. Even the octet guideline (that I personally like to refer to as a rule) is not as strict a rule as proper physical rules.

As Ben points out, in diiron nonacarbonyl iron(0) has a coordination number of six, which immediately voids your ‘excitation hybridisation’ argument. I am unaware of any fully characterised tetracoordinate iron(0) species. However, I guess they exist due to bulky ligand design and that they will be either square planar or tetrahedral in nature depending on the type of ligand in question. The type of ligand and its interaction is much more important than a hypothetical ‘excitation hybridisation’ scheme.

Please forget any and all of the wrong things you learnt about hybridisation with respect to transition metal complexes. Please refer to molecular orbital theory to explain these instead.