Ian,
Yes, you can run ULT sound file locos with non-ULT locos. The non-ULT locos will notch up on queue with the ULT locos when the ULT is leading. This will occur even if you have the follower locos in manual notching....they will automatically change behavior to match the lead.
You can also use a non-ULT sound file with all the new features, including load and brake. The prime mover will ramp up as expected under load. It will not be as smooth as the ULT files by nature, but it is certainly usable. I have tested this will several of the old sound files and it works fine.
-Kevin