RF trace matching network component pad sizes
The most common approach is to design your microstrip or CPWG to match the component pads for devices in the path.  For 0402 components, that means 20 mil trace, as you mentioned.  And, yes, this means generally using all 0402 components for that RF path.
If you use narrower trace (12 mil) with 20 mil pads, you will have unwanted parasitic capacitance at each pad location.  Depending on the frequency of operation, this may or may not be a problem.  Frankly, for BT at 2.4GHz, this may not be much of an issue.  Still, why settle for less than the best you can do if you are designing the PCB?