I pronounce
Cosima as coh-SEE-mah and I think it's definitely a usable name. My first impression of a person with this name is that she is intellectual, thoughtful and very kind. And she likes classical music, for some reason... (
Cosima Wagner was the wife of composer
Richard Wagner.)
I made a poll of combos with
Cosima a while ago, so there are my middle name suggestions for you:
http://www.behindthename.com/polls/244845