The  black enclosure is "separate" module.  It fits between the non modified smoke alarm and the mounting base.  The ESP-12 should work well.

Hey Matt. Like your project and I want to duplicate with ESP-12 board (all in compact board).

Would it be just as easy to make a separate module that plugs into fire alarm network and not take apart smoke alarm at all? I also have Kidde wired smoke alarms so should be easy following your example.

thanks

Gary