Here some background information on real-world turboprops that doesn't apply exactly as described to X-Plane, but provides valuable background information for a designer using X-Plane's Turboprops.
Below is an example of free turbine engine (CT7 series turboprop engine Saab 340).
CT7 Turboprop Engine sectional flow diagram. This illustrates the "free turbine" concept. The compressor section and first 2 power turbines are not connected to the last 2 of the 4 power turbines. Therefore, the 2 last power turbines turn independantly of the rest of the engine. The propeller (prop gear box) is connected to the last 2 power turbines by the solid central shaft.
There are typically two types of engine controls for a turboprop engine. Some have throttle, propeller RPM and fuel (condition) levers. On this type, the condition levers control the amount of fuel that enters the engine, by controlling the Fuel Control Unit (FCU). Others have control levers (throttle and blade angle) and condition levers (fuel and propeller RPM combined). See diagrams below for Saab 340 example of control/condition levers.
Accordingly, the panel requires slightly different sets of instruments for each of the turbine types.