The question "Do the sounders have to be compatible?", can be reworded as "Do the sounders have to work with the panel, and with the other sounders on the circuit?" Compatibility is your assurance that the sounders (the horns, chimes, etc.), and the strobes will work as advertised.
Over the years, besides concerns about voltage and current for all Notification Appliances, like sounders, horns, strobes, chimes, speakers, etc., synchronization has been developing as a major compatibility problem.
Each manufacturing brand of sounder (fire horn, chime, etc.) and each brand of fire strobe is synchronized using a different method. If the sounders are not synchronized, the temporal sound or the march-time sound from several horns would overrun each other, and the occupants of the building wouldn't be sure that it's a fire alarm.
If the strobes aren't synchronized, worse things can happen. It has been shown that three flashes per second can cause epileptic seizures in a few people. If, during a fire alarm, there are three non-synchronized strobes visible (flashes reflecting off walls counts as the strobe being visible), someone might have a seizure and not be able to get out of the burning building.
To prevent these problems, all fire sounders and all strobes have to be synchronized - the sounders have to sound-off together, and the strobes have to flash at a one second rate - together.
That's one place that compatibility comes in. Because each manufacturing brand of horns and strobes use a different method of synchronization, all horns and strobes have to be compatible with the control panel they are being used with.
Mixing and Matching Brands
The compatibility issue is even deeper. Some manufacturing brands of control panels and many brands of NAC power supplies can be adjusted to they will work with other manufacturing brands of synchronization. When the control panel or NAC power supply is set for a specific brand of synchronization, only that brand of sounder or strobe can be used on the circuit. When adding sounders and strobes to an already existing fire alarm system, ANY sounder or strobe using another brand of synchronization will not work correctly, if it even works at all.
Just asking the technical support team for the panel's manufacturer "Can I use this brand of sounder or strobe with your panel?" may get the answer "Yes, you can use it".
However, asking the question "Can I use this brand of sounder or strobe on your panel at the same time as another brand of sounder or strobe is on the NAC circuit?" and the answer is probably "No, the panel can work with one brand or with another brand, but not both".
Stick with the manufacturing brand that's there. Anything else is not compatible; anything else probably will not work. Even if the added sounders or strobes do sound off and flash, they will not be compatible and they will not be synchronized with the rest of the system.