yes, and the stoics would say that you are morally required to take political action (if your intention is rational and pro-social), this is a core component of stoicism and what differentiates it from epicurianism (which is not the pure hedonism that many people think).

The stoics would say that you simply shouldn't have desire or attachment to the external result of your actions, rather than your intentions and actions themselves. They simply are not disturbed if their actions do not work out, instead they take fulfillment in being a person who pursues justice.

The layman often equates the meme comic with the dog in the room on fire saying "this is fine" with stoicism. If the dog were actually a stoic, he would be calmly but efficiently doing everything within his control to put out the fire.

wrt slavery specifically. In the Greco-Roman times slavery was thought of much differently than we do now. Many have wrote about this topic (mostly a lot of theologians and apologists) rationalizing slavery in the ancient times, but the truth is I just don't think we can simply relate to it. Obviously we've moved on and any rational person considers slavery horrible. But thinking about history begs us to question what do we think is OK nowadays, that future generations will think is horrible. I'm certain there is something.