Yes and no.
The Elk will work fine with RCS zone controllers installed. It just doesn't talk to them. I know, sounds like a funny statement but here's what I mean:
1) The RCS thermostats and zone controllers all operate on the RS485 network.
2) The Elk talks on this RS485 network but talks directly to each thermostat.
3) After the Elk gives the commands to the thermostats directly, the thermostats then communicate to the zone controllers.
This is exactly how we installed the Elk with the RCS thermostats and the zone controllers in the North Texas HA Users group project house. In that house, there are 10 thermostats and 3 zone controllers. Everything works great. If you think about it, there really isn't any reason why the Elk has to talk directly with the zone controllers.