Like with any airport, gates can be subject to change. Honestly, I am not 100% sure SXM has any "fixed" gate for any airline. I have departed both upstairs and downstairs on both USAir/American (now) and Delta. I have also seen the same on other airlines leaving there. The type of plane can also play a part in where they park upon arrival. None of the gates are too far apart so upon check in you should be able to map out a strategy.