I consider Cassidy a girls name (well, a surname, but after that a girls name) so I would go with Carey first (love the Joni Mitchell song) and then Gene (because Gene Hunt is awesome).
Leo and Sadie are brilliant too and were both on my shortlist for both DD and DS (in fact DS has Leonard as a middle name because we really liked the name, but we liked something just a smidge more for his first name)
You've set the bar high with Leo and Sadie, lovely!
Hhm, my initial thought is that Cassidy feels like a modern girl's name along the lines of Maddison. Can't decide what I think of Gene, I think because it sounds exactly the same as Jean it might store up trouble.