Sorry for my ignorance, but the chiller you are describe have an outlet on the chiller for heater control?? I remember one company does that. If it has it, use it.
My chiller is a JBJ unit that has worked great for years, but the cooling cycle is +- 1 degree. If you set it to 79, it comes on at 80 and shuts off at 78 so there is a 2 degree swing. I didn't like that, but after talking to several refrigeration guys it appears to be the norm. No problems with the fish/coral.
The best thing is always a separate controller. Run the heater on the controller and let the chiller control it self. The chiller may not like the constant cycling on/off if hooked to the controller. A Rancho controller is not that much when you consider the cost of your wildlife. R