Chrigel,
One thing you may want to consider is the limitations of X-10 codes. There are 16 house codes with 16 unit codes each, 256 codes that are available for use. If you wanted to use X-10 controls on the thermostats, you need one house code per thermostat. In a 6000+ house, I am sure you have more than one A/C unit.

Another problem to look for is multiple electrical panels. Since X-10 signals are transmitted over the power lines, two panels in the same home can cause some problems with signals not getting from one side to the other. This can be overcome with couplers but this application requires some unique wiring techniques.

Chris Couture