To just the author and mods, unfortunately we cannot support this yet, as we don't have a "post author" pseudo-group. That is a good idea, though.
We did accomplish this via nodebb-plugin-support-forum, you could try this and see if it still works?
(Suggestion) Moderators have no limit when chatting
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Hi, I will be glad if you share your comment about this suggestion
https://github.com/NodeBB/NodeBB/issues/6303
