I think everyone else has given you great suggestions, so these are my favorite options....
Allegra - I ADORE
Allegra and can actually see myself using "Legs" occasionally so maybe
Lex /
Lexie could work? Have you considered
Allegria? Then you could use "
Ria"
Calista - Liss or
Lissa feel most natural to me. That could turn toward
Liz I suppose.
Juliet -
Etta, Uli ("OO-lee"). I like
Letty best though.
Sapphira -
Iris or Irie maybe? Blue or Gem if you wanna go real abstract.
Valentina - this one's especially tough.
Alen is all I got, but I've always loved
Allyn on a girl.
Seraphina - I like Phi best because I already love Fiona/Fi and Phoebe/Phee. Or maybe Sina ("SEE-nah")