An Adaptive Part is "just a part". If you place it in Assembly A at one length, then place it again in Assembly B at a different length, you are still only placing a single part which can't exist in multiple states in different assemblies.
"Flexible" only applies to an assembly - not a part - so you can't apply the same logic as Adaptive. (Ok, "Flexible" does exist in Sketch Blocks, but that's different.)
But I totally get what you are after. Have you tried "Place iLogic"?

This works for Parts and Assemblies - and if you forget the word "iLogic" for a second, all this does is place a copy of the selected component into your assembly. You can include adaptive features (not cross-part adaptivity because of the dependencies involved, but certainly adaptive features) if adaptivity is your thing.
The biggest advantage of "Place iLogic" is that it takes the original as a 'template' and builds you a new 'unique' part - and you can mess with it without breaking any other assemblies where you've used the same component.
Peter