Question:
I'm installing several smoke detectors in my house, all connected so if one goes off, they all do.

The detectors need a wire (other than the hot and neutral wires) to connect them so they can signal each other. Can I use the ground wire (the bare copper wire included in standard 12 guage romex) as the connection wire.

I hope the answer is yes since I already roughed in the wiring

Answer: No, you can't do that, because then you don't have a ground (that wire must be connected to the system ground in the panel box).

The smoke detectors should be on their own circuit (so that a circuit breaker that trips for some other reason doesn't cut the fire protection). You need to run a cable with three conductors plus ground and use the third conductor as the signal conductor.

Smoke detectors are an extremely light load, so I used 14-3 cable rather than 12-3 (because 12-3 is a royal PITA to run throughout a house) with a 15 amp circuit breaker. I use 12-2 for all my other circuits (all on 20-Amp breakers).