I like
Seraphina; it's not my favorite name ever, but it's elegant and unusual.
I prefer
Seraphina over
Serafina. The ph just looks more balanced and natural than the f, and I agree with you about the
Matteo combo. It might be a little problematic if you have a long name, but practice sounding it out and maybe it'll work and maybe it won't. I guess only you and your husband can decide that.
I know a
Seraphina who goes by her full name, much to her on chagrin, and she keeps insisting that she be called either
Sera or Phina but her parent refuse to let her go by either. I think
Fifi would also be a logical nickname but not one that I would ever choose ;-)