Oooooo, fun! Here goes...
Magdalena:
Maggie,
Dalia,
Lena,
Nana, Doll
Sofia: Sof,
SofieLarisa:
Reece, Larry(if she can take it; I suppose I should explain that I am a fan of male diminutives on females)
Josephine:
Josie, Fin(ny), Seph
Anika: Ann(ie),
Nika,
NikkiAnastasia:
Ana( or
Anna or
Annie),
Stacy, Sia(something with that sound),
Asia
Gabriel:
GabeEmanuel:
Manny (not a fan), Eman (or something like Aemon)
Sebastian:
Bastian, Sev,
IanDominik:
Dom,
Nick (or nikko, nikki)
Simon: I don't have any good nns for this, maybe Sigh?