[Stiglr]

All that depends on the specs for your end product. Check within your X-Plane community for tips, if not some specific poly limits.

Simulators that I have experience with establish separate poly limits for exteriors and for interiors, all of that hinging on how the game operates (e.g., where is the player's point of view during game play, how close objects can be seen [whether in interior or exterior or chase view], whether a plane requires "level of detail" versions for viewing at distance). There are a lot of things to consider.

To answer your specific question, yes, it would be possible to "mix" low and high poly modeling for the exterior vs. the interior... but your decisions would likely still be limited by the same in-game performance issues.