Tau expand between every codex. Shadowsun also led the last expansion so I'm not sure this is that much news. Having a book about it though is going to be great.
And it's going to be a long time until Tau seriously annoy the Imperium, they've swallowed a few fringe worlds and are rubbing shoulders with Macragge, but in the prior case the Imperium doesn't care enough yet and in the latter case, the Tau and the Ultramarines are pretty much permanent allies so there shouldn't be any major diplomatic incident.
There's a reason that Tau and Codexarines are Blood Brothers in the ally chart. I'm really not expecting a whole lot of strife, honestly.
I'm really concerned with how the Riptide will compete. It feels like it could easily be a Heavy Support with its firepower... but it's not competing with Broadsides or Hammerheads. Will it be more effective than 3 Crisis, however?