You need to define all four items including the group:
Group gDownstairsThermostat "Downstairs Thermostat" (gFF) ["Thermostat","Fahrenheit"]
Number DownstairsThermostatCurrentTemp "Downstairs Thermostat Current Temperature" (gDownstairsThermostat) [ "CurrentTemperature" ]
Number DownstairsThermostatTargetTemperature "Downstairs Thermostat Target Temperature" (gDownstairsThermostat) [ "TargetTemperature" ]
String DownstairsThermostatHeatingCoolingMode "Downstairs Thermostat Heating/Cooling Mode" (gDownstairsThermostat) [ "homekit:HeatingCoolingMode" ]
You can do that but it is not necessary for the skill to work.