Fowlishness's Slytherin, Irene, could be his connection, including the Quidditch and Dueling. And if Arlear is down, despite the Ravenclaw/Slytherin hatred, they could've become friends by pranking one another over the years, or frenemies if not friends.

And for dueling club, Nym's not part of it, seeing she's more of a flight not fight. Dale is too air-headed/hippy about it too, although she can be very good when she puts her mind to it. Lydia could be friends with Veronica or someone associated.