While I have seen conventional zones that mix 2-wire smoke detectors, 4-wire smoke detectors, mechanical heat detectors, and pull stations, and when tested, they seem to work, it is not recommended, and goes against most manufacturers’ instructions and probably several code sections.
The issue is that 2-wire smoke detectors operate on a different principle from other initiating devices. Pull stations, 4-wire smoke detectors, and mechanical heat detectors are all basically a normally-open switch. When they go into alarm, they short one terminal to the other, and the panel reads this short on the IDC as an alarm. Some older panels only work with this type of initiating device and cannot be used with 2-wire smoke detectors.
2-wire smokes are similar in principle, in that they place a load, or almost short the two IDC wires when they are in alarm. The different is due to the fact that 2-wire smoke detectors also draw 24 volts DC power from the two IDC wires. If they were to completely short the two terminals, they would bring the voltage of the circuit down to 0 volts and lose power. So a dead short on an IDC from a pull station, heat detector, or 4-wire smoke detector will cut power to all 2-wire smoke detectors on the circuit.
This is not a huge deal, but it can be an issue in the case that there are multiple devices in alarm on a circuit. Say, for instance, that a two wire smoke detector goes into alarm, triggering an alarm at the panel. The LEDs on this detector will light red, as will any remote annunciator LED or integrated relay contacts. If a pull station on the same zone is pulled, the detector and annunciator’s LEDs will go out and any relay will reset. Also, some panels have IDCs that must be configured as either 2-wire smoke detector or pull station/heat detector/etc, if they receive an alarm from the wrong type of device on that circuit, it may damage the panel.
If you do mix them, don’t wire them as Lambda said he found, that is definitely the wrong way to do it. Connect the + and - of the 2-wire smoke detectors to + and - of the IDC. When you add a 4-wire smoke detector to the circuit, connect the two alarm contacts to the IDC wires, polarity doesn’t matter. Connect the + and - power terminals to + and - resettable 24V power from the panel, not an IDC.
As far as reversing polarity goes on this mixed zone, you can reverse polarity on the IDC wires and it will trigger the sounders on only the 2-wire smoke detectors. The 4-wire smoke detectors won’t know the difference though. To trigger the sounders on the 4-wire smoke detectors, you must reverse polarity on the 24v resettable power going to them. Sound confusing? Stick to one type of detector per zone, I promise it will make your life easier.