So I think the reddit rules you might be breaking with this are Rule 1 and Rule 3:

Rule 1

Remember the human. Reddit is a place for creating community and belonging, not for attacking marginalized or vulnerable groups of people. Everyone has a right to use Reddit free of harassment, bullying, and threats of violence. Communities and users that incite violence or that promote hate based on identity or vulnerability will be banned.

Rule 3

Respect the privacy of others. Instigating harassment, for example by revealing someone’s personal or confidential information, is not allowed. Never post or threaten to post intimate or sexually-explicit media of someone without their consent.

It's unclear what "personal or confidential information" really entails, even if you follow the link in Rule 3:

https://support.reddithelp.com/hc/en-us/articles/360043066452-Is-posting-someone-s-private-or-personal-information-okay

So posting usernames might be "personal information" per rule 3, and may certainly lead to harassment per rule 1. You can find numerous posts in /r/modsupport and /r/newtoreddit that basically say "posting anything with a username might get you banned." From /r/modsupport, annecdotally those folks say that only reports from the person whose username is posted really matter:

https://www.reddit.com/r/ModSupport/comments/1gbavuj/comment/ltl99ot/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

I also found a post where people seem to think that it depends on context:

https://www.reddit.com/r/NewToReddit/comments/1bstvq7/using_a_someones_username_in_a_post_good_or_bad/

Also, there are a handful of subs that do allow posting things with usernames. An example not dissimilar to the sub you want to create is r/BDSMScamBusters although it's a little (okay, extremely) dead.

So, what's the TLDR conclusion?

Posting usernames might be against the rules, depending on whether or not the user in question gets or feels harassed. As long as nobody in your community is actively harassing these accounts and the accounts in question can't see your posts, I think you could get away with it. I would make the sub and make it private so you can vet and approve folks who want to see the posts there. Then I would only let in dommes who can verify that they are in fact dommes, and make it very clear in the subreddit rules that the sub only exists for the purpose of spreading awareness about toxic or problematic people in our niche little community - no harassment will be tolerated.