edit* captured settlers from another civ wont return as settler once recaptured, they remain slaves.

If a settler has been captured, even in vanilla, it loses its "settler" status and is instead a worker, even if another civ (like you) recaptures that worker, it remains a worker. This is unless it was barbarians that captured it. Then it remains a settler until you either rescue it, saving your settler or another civ captures it, in which case it becomes a worker (or should be a slave with this mod). You should also get the option to return it to the original civ (for diplo favor) if you capture it. I think even in this case it still remains a worker though. So, even in vanilla, you can't straight up capture a settler from another civ. You should still be able to rescue your own from barbarians and get a settler back instead of a slave, unless someone else got to it first. If that's what's happening, then that should be ok, or at least in the spirit of the original game design. I think the idea behind this was that being able to just nab someone else's settler (and use it to settle your own city) was OP. But also it punishes you for not getting to your settler before someone else did, which I also think is reasonable.

As for getting slaves from city state tributes, I don't know if anything about that was changed for this mod. So you would still get a worker as in vanilla. To get slaves from capturing cities I'm pretty sure you need honor. I don't know if having honor or not would make any difference to tributes.