Yeah, I have no problem with Nixie and Tristan trying a relationship at all, it'll be even more interesting to see not only the interactions between two mermaids, how they react to each other and how their powers will react to it as well being complete opposites.

Also, I have no issues inviting someone else to join in as a forth member, but even if someone doesn't join straight away I can bring in one of/ all of Tristan's brothers as more than just background characters; considering they all surf and do water sports they would be harder to keep the secret from/ have a good chance of turning if the opportunity arises.