Willow,
Savannah, and
Tristan seem to go together in my mind because they sound very rich, while
Kai (
Nikias) and
Aria (
Ariella) just seem too short and common for the other names. (I think
Finnegan goes well with
Willow,
Savannah and
Tristan, but not the nickname
Finn. It's possible that I just think that because I had a horse named
Finn and everytime I see "
Finn" I think of a horse!)
As for twinsets I would go with: Willow/Savannah, Savannah/Tristan, Finn/Tristan