Are there restrictions on what I can do with these materials and textures?
No, there are no restrictions what so ever! I believe that the power of this addon is in it's versatility and it would be quite a bit less versatile if you couldn't share things you created with it. The only thing I ask is not to put the individual InstaMats on their own up for download somewhere, but this is just a request (uploading a model with InstaMats on it is of course fine!)