I'm not 100% sure but I started looking into this myself a while back (never did it though) and from the research I did, I believe that not all smoke alarms operate on the same "trigger voltage" to send the signal to the others, so I never tried putting just one in and leaving the remaining others in place. My detectors are all about 8 years old, so in 2 more years I plan on upgrading them all to the GE 350's with the one in the basement having the relay contacts to trigger the ELK. I have FIREX throughout the house as well and looked into the firex relay, but due to the issues of it not working while on battery power, etc, I just postponed this project until I replace all of the smoke's at once. You might want to do some more research though to see if that is the issue (the voltage sent between the smoke detectors).
I just did some quick searching and pretty much every link I found said don't mix and match smoke detectors...I'm sure you'll find the same info via Google as well. Again, this is why I postponed it until I replaced them ALL at once.
