Tomorrow morning the kitchen is being painted (last room to be painted) and the smoke detectors, new thermostat, and curtain rods are going in. 

If you don't provide them one, they're on their own.I have found A/C units to be a hassle, especially when tenants turn down the thermostat too low and freeze up the unit (or break it).How much is your insurance deductible, OP?

The tenants want to install security system with smart thermostat adapter and one main keypad?

You don't need to do anything with the furnace other than if you still feel very uncomfortable or worried about freezing; leave it on and put the thermostat low around 40 - 45 F.

Next time I would add a wall mounted thermostat to control the heat cycle.You could run CSST to the firebox but I would advise a black pipe interface within the firebox as it gets very hot within.

They could be setting the thermostat up but I think not.  

The other thought is I could give them all control of the thermostat, leave it in the common area, and let them all collectively set the temp, but that seems like a recipe for war.

Hi Jake,I use a thermostat from Chicago Controls that keeps the heat set at 68 degrees.  

The thermostat is located in one unit to control the heat for the entire house.

I am not taking the Habitat store, I am talking real business furniture dealers that have used stuff, often good things to have- Efficient heating system, depending on age, plus fancy thermostats that can limit the range they operate in (or just a lockbox).