How can I replace the imp officer model with another one? In particular a clone trooper model, all from the base game. I downloaded free Hex editor so am I able to use that? If so. TEACH ME PLZZZZ

Find the model and texture in rep\msh, and copy them to imp\msh.
Then change the appropriate lines in the appropriate unit's odf file.
Sorry if I seem terse, I'm writing this on my phone.

I don't know much about hex, but since your new trooper is in a complete different file and not just an different reference (for example replacing the engineer by pilot) I don't think it is possible.

so i'd rewrite the stock missions and load the commander from the rep lvl