There are three pimps in the district but you only have to deal with two of them. I’ll be listing the two pimps that will lead to objectives with good reward and damage.
The pimp, Billy Zacarro is located on a warehouse to the north. You don’t need to kill everyone; just wait for him to finish the conversation with another goon, kill him, then position yourself behind the nearby crate and lure him by whistling. That way, you should be able to interrogate him without alerting anyone else. Otherwise, take out the sentry first then immediately disable the pimp to prevent him from running. Take out the other guards then subdue him.
The pimp, Pietro Lovine is an easy target with just a handful of guards and one sentry. Prioritize taking out the sentry first, which is a lot easier to do with silenced weapons. Stealth kills is recommended here to save you time and trouble instead of chasing him if he gets spooked. Furthermore, there are cash stacks that you can collect from the rooms in the second floor so being able to collect them while also accomplishing your objective will be a lot better.