Danielle for me is one of those overlooked, inoffensive but lacking any real personality names; French but not TOO French, you know. I grew up with a LOTTTT of Danielles, Michelles, and Nicoles, so to answer your question, a
Danielle can look like anyone, be anything. It's a blank slate name.
Sure,
Joy works as a middle name, that's the great thing about those one-syllable middles, they work with darn near anything.
Danielle Joy is a perfectly acceptable name, it works well,
Is
Cassidy a boy or a girl in the sibset? I think
Dani and
Cassidy work, and
Dani and
Cass is really cute, but
Danielle is just a bit too frilly next to
Cassidy, it makes it ambiguous.