Can you run an individual wire to link smoke detector together?
Even if the wire ends up only having a low voltage on it, it will be terminated and spliced in the same electrical boxes as the white and black high voltage wires, so it's going to come down to the same rules as any other wire in the circuit.

Using 14/3 is the only right way to do this, unfortunately.  There is an exception for running a ground wire along a different path than the rest of the circuit, but that wouldn't apply here.