Connect the sensors in series most 12V smoke and CO sensors have an interlock connection to trigger all devices connected to them and you would run them in series with each other to obtain this.

I would use the security system 12V AUX output to power them (same with the smoke detectors). You already have a battery backup on your security system so why add more than needed?

You can program the zone to be 24hr Gas which triggers the alarm no matter the state of the security system. You also want to change the siren to be pulse not steady (same with the smokes).