I posted a response to this question yesterday:
It is a perfectly valid question, the OP is asking if different models of Yongnuo flash will work together, or if they would need to be all the same.
Then it was marked as duplicate, the duplicate being this question:
The proposed duplicate doesn't contain anything at all to answer the OP's question. I couldn't believe my eyes quite frankly. Is it just OK now to mark everything as duplicate without even reading the details?
I admit that the OP started asking about how they can achieve it, but in the description was asking if it was possible. The OP however is specifically asking about only Yongnuo flashes working together.
The proposed duplicate is asking if it is possible to pair a Yongnuo flash with a Nikon SB900.
Furthermore, there are certain scenarios where the extra Yongnuo flashes may or may not work, depending on the setup and none of that is included in the duplicate, regardless of Nikon or Yongnuo, apart from a little bit about TTL.
It's this kind of thing that annoys me about SE. It's clear that someone marked this question down and then all the robots jumped on it. The rules say that in order to have a question re-opened, that you must explain why the suggested duplicate doesn't answer the question. I think it would be better if every person who marked it as a duplicate should clearly explain why they think the suggestion provides an answer, clearly citing all references to prove that they didn't just mark it down because someone else had, or because someone was an inexperienced user (which also happens too often).