How do you wire the above units so the smoke/co detectors will all ring when heat detectors are activated. The heat detectors are a two wire hook-up. Existing wiring to heat detectors is existing and in wall. Hard to change to 3-wire.
You'll need to run a /3 cable from the smoke alarm run to wherever you want the relay to go, and a /2 cable from the heat detectors to the relay location, which will need a box at it (a simple 4" square box will do if you aren't putting the relay at a smoke or heat detector location).
Once that is done, connect one wire of the /2 cable to the red wire in the /3 cable, black and white on the relay module to the matching wires on the /3 cable, the grey wire on the module to the other wire in the /2 cable, and all grounds together and to the box the relay is in, if it's a metal box that is.
This way, the 9V supply from the SM120X will be connected to the red interconnect wire when the heat detectors close, causing the smoke alarms to sound. Note that the heat detectors will only work if there is 120V AC power available to the house, even if the smoke detectors themselves have battery backup. (The SM120X doesn't provide a backup battery for its 9V supply, in other words.)