Can you mix brands of hardwired smoke detectors?
Likewise, can different brands of smoke detectors be interconnected?
You cannot interconnect different makes of alarm, they have different signalling. Buy one of the same make and don't mess around. Smoke alarms provide a degree of safety - only fit and use them in accordance with the maker's instructions.
Thereof, are all hardwired smoke alarms the same?
Most hard-wired smoke detectors have a connection from the electrical wires to the smoke detectors that engages and disengages. It's like a plug. The key is knowing what connector your smoke detector has so that you buy a new smoke detector with the same connector.
Hardwired simply means that the smoke alarms are physically attached to your home's electrical wiring. The term is used to distinguish this type of smoke alarm from the less expensive, more easily changed smoke alarms that adhere to the wall with double-sided tape or are attached with screws and that run on batteries.