Most (probably all?) tournaments require you to pick your race and keep it for the whole tournament. No switching based on map or matchup. But you can pick random.

My amateur impression was the following rankings:

Easy to hardest to play: Protoss, Zerg, Terran

Best when mastered: Terran, Zerg, Protoss

I wasn’t tapped into the pro/competitive side so I’m sure it played out differently for them.