As the holder of a foreign ATP, FAR 61.153(d) allows you to bypass holding a FAA commercial certificate. As Klimax says, you just have to pass the knowledge test before the flight test.
Also, 61.23 (b) says you do not need to hold a medical to complete the checkride if it is done in a sim, you only need the medical to fly the airplane. Some training centers may require you to hold a medical so check with them.
I'd say Sheppard. You're not going to learn the material but it fills the square of passing the test.