It was fairly obvious it would be her.Plus, marrying a slave would be pretty iffy, consent-wise.
Nah, if he married Carla, she wouldn't be a slave anymore. Which is in line with Souma slowly abolishing slavery. That and her slavery was never meant to be for life anyway, her father, which had a higher crime, was made a commander of a special unit, Carla marrying Souma really isn't far fetched.