This would push users toward local posts (especially on smaller instances) while still supporting a better distribution of user engagement over the threadiverse. And if you don’t trust that it was implemented correctly – you can simply check it by counting the amounts of posts against your chosen probabilities.
Edit: changed feed name to mixed-feed
When someone creates a new discussion in a merged community, which servers would it be posted to? All? Of only one of them, wouldn’t it be confusing for the user to suddenly have to choose? What if the component communities have different moderation policies?