Ron

I have Hessens- I think they run a tiny bit smaller, especially in the toes.  I normally wear a 10 Mens US, and I got that foot size (custom for the calf size, but foot part was standard), and they're a bit snug in the toes.  I would recommend going a size up for that if you have a wide foot.  
 
Hessen boots will go L3- I made L3 with them.  Pretty much all Officer Boots have the same CRL text- they go across many different Officers, that use the riding boots (some use Jack boots).  So yes, Tarkin and Line Officers use the same boots (as do Staff Officers).  

Next to most other Line Officers the collar of Peter Cushings uniform seemed to be a little bit higher than on most other uniforms. But that's only a minor detail.

Tarkins uniform is exactly the same as the Line Officer uniform. All you need is a white collared shirt underneath and that's it. And look gaunt as mentioned above. 

As far as I’m aware that’s pretty much it!
You’ll also need to make your eyes and cheekbones look “gaunt and darkened.” Hopefully they weren’t saying you had those features already!