Hi Todd,

At first you need to look at this AKN article:
https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Pasted...
I believe it will help you to understand what happens when you copy - paste objects between drawings with different units format.

I think the best way for you will be to convert units in one of the drawings you have (depends which units you need - prefer) and then proceed with copy / paste from one drawing to another.
Here you can find additional details about -DWGUNITS command:
https://www.cad-notes.com/convert-autocad-units-using-dwgunits/

>>>>but when I try to measure a wall or roof length, the lengths again are meaningless
How do you measure length? Do you use DIM command to create dimension or you use DIST (or Distance option of MEASUREGEOM command)?
If the first, then you will get length according to the settings in the current dimension style, which may differ from the result you get with DIST command. DIST command always gives you distance in AutoCAD drawing units.