You might want to note which one they are installing, however since it is 18/4 it appears it is a 4 wire smoke detector. I believe the 'limitations' in the form of needing supported smokes only affect 2-wire smoke detectors.
If this is correct (I don't remember hearing about needing specific brands/models for 4-wire), then you should be fine. This assumes they are four wire smokes though, if they are the 110 ones then I dont think you can do anything. If they are two-wire then they need to be on the compatible list. It also isn't clear what the ones they put in connect to.
I went with two wire smokes from System Sensor, but after installing them, if I were to do it again I would have used 4-wire smokes. Just seems to make more sense after having gone through this.
This all does not concern you however. Back to your question, if all else fails and running the wires is a problem, there is always CADDX based wireless smokes that can integrate with the Elk assuming you add the wireless expander (which if you do, don't forget to order one of these:
http://www.automatedoutlet.com/product.php...&cat=124&page=2
Hope this helps.