You are feeding a three phase secondary load and will have to have a breaker protecting the 208Y120 panel. I'd go with a back-fed 60 three pole main breaker.
Don't forget to use bonding bushings between this panel and the transformer. You'll want lay-in lugs. Use a four barrel mechanical lug on the Xo or at least one double barrel lug plus singles.
Size your grounding conductor according to 250.66 not 250.122. (#8 minimum)
Consider placing the 15kVA XFMR overhead/above and adjacent to the panel so as to save on space.
Vibration hum can be addressed by isolation pads.
