If you mean can you have a standalone conventional detector or detectors and install Nest Protect separately, then yes. However, if one of your Nest Protects sounds an alarm, your conventional detector will not sound unless it also detects the issue and vice versa if the conventional detector senses something that Nest Protect hasn't yet.
If you mean interconnect Nest Protect with a conventional detector then no and that's pretty standard. Each manufacturer uses proprietary detection algorithms meaning there’s no industry standard. What that means in practice is if alarms from different manufacturers were to be connected together, they may not warn you properly in an emergency. The NFPA actually prohibits the connection of alarms from different manufacturers without special testing. As far as I'm aware, there's no manufacturer that sells a detector that is designed to interconnect with a device from a different manufacturer and some manufacturer's even state in their documentation not to try to connect their device to that of a different manufacturer.
