He says when he plugs new cluster in it reads the right mileage already. I see the menu has cluster replacement on some of the immo systems listed. Always sounds like a do or die on these things. 

SSM3 would do it for sure.  Autel 608 might be able to do it under the diag side. Make sure you get the mileage and configuration data correct, you only get one shot at it. 

Used car guy called and is looking to have his new used cluster paired to his 12 crosstrek. Car still runs with old cluster but gauges are faulty. he plugs in new cluster and car will not start. What does it take to pair the the new cluster?

run the function for replaced PCM under the immo section It will ask for the pin number for the car. Will write the VIN + Pin to the ecm and it will run.   Run the part number + VIN thru a friendly dealer and see if its the correct part unless you have the original to match #'s     Lots of autopart stores and wreckers selling wrong chrysler ECM's in my experience (Most notably the 3.7 and 3.8 which have different firing orders)    

If they matched the Alpha Code it is do-able with the 608. Overwrite the VIN in the donor