Does the Free Space Path Loss model apply to all types of electromagnetic waves, including light?
Your first paragraph answers itself. 

Also, if you want to create a comms system using (any wavelength) of light you will need to start with the photodiode or phototransistor that is used to receive the signal and work backwards to the transmitter.

A photodiode will have a sensitivity i.e. so many amps per radiant power hitting it. You can work out the actual power by considering its effective sensitive area. You also need to consider how small the current signal can be that is needed to get good reception of data. For this the photodiode will have a noise-figure.

So, you decide what current you need to flow that represents reasonably clean data and work back to power received in the sensitive area of the photodiode.

That tells you how many watts per square metre you need. The distance between send and receive and the beam angle of the sender then allows you to calculate the output power needed from the emitter.