This is not a bad idea, but be aware that, like flexwatt, RHP's do NOT heat the ambient air temperature. They work using radiant heat which is directed downward and heats the surface immediately underneath the unit. So if your ambient temps are too low, an RHP will not help you - with or without flexwatt. The AIR temp will not be affected to a large degree at all. This is why I suggest the space heater for the room - that will adjust the AMBIENT temperature to the appropriate degree.
Additionally, you can buy a ceramic space heater for around $50-60 which will heat the entire room and all the cages within that room will be positively affected by it. A single RHP will run you roughly $65-85 and you'd need one for each cage. A space heater and an RHP both cost pennies to run if regulated by a t-stat. The amount saved off either unit in its operation is negligible.![]()