To do something different for "first posts" and "reply posts", you can react to the Flarum\Post\Event\Saving event and do something similar to https://github.com/flarum/core/blob/v0.1.0-beta.10/src/Post/Command/PostReplyHandler.php#L81 to check if $discussion->post_number_index > 0.
That's definitely not the easiest thing to extend. To change the actual validation rule you probably also need to register an alternate post validator in the container, or replace the whole PostReplyHandler/StartDiscussionHandler via the container as well. It could break other extensions that will attempt something similar.
