I'm trying to create a new model with the personality_sphere_skins as the base of it.

Upon recompiling and testing it in the model viewer, it spits out that it's not a valid model.

Could somebody please look at this?

Taking a look to the .MDL file, it seems that your model needs the materials here:

materials/models/npcs/personality_sphere/personality_sphere_custom/

Not here:

materials/models/npcs/personality_sphere/

Maybe this makes you not to be able to see the model.

But if the error you are receiving is related to the model itself, I won't be able to take a look into it until I arrive home. However, why not just hexing the model instead of decompiling-recompiling it?