I agree with Benito.
In fact, I think that “Casual” brings the perfect forum for this.
In “strict” rengo, I think that rengo rules, including no talking, should be expected.
In “Casual”, I think you have the opportunity to define it up front, and as long as all participants agree and are not suprised, you can do what you like.
OK: agree with everyone before you start (in the chat window in the team management pane) that chat is expected
Not OK: chatting about the game after it starts without agreeing beforehand.