If I buy a 160mm Deore rotor, can I fit it to replace an XT one?
They aren't exactly the same design but you can use either rotor in either brake (as well as many others)
The difference between 6 bolt XT and 6 bolt deore is XT have more holes drilled in em to make em lighter - totally interchangeable tho!
You don't need to stick to Shimano either, rotors are rotors, as long as the diameter is the same, generally speaking. Cue someone to point out problems! You just change the calliper adapter if you want to change rotor diameter.
Only compatibility issue I can think of are rotors with riveted aluminium centres (eg Hope) can fowl pads on some callipers (eg Hayes). But irrelevent for you.
In this case, you might consider rotors from someone else, they can be cheaper and lighter than Shimano, eg: Superstar or Ashima Airotor from Chain Reaction Cycles.
BTW check my classified listing 😉