other forms of John include Sean (there is no j in irish--and its easy to see how the french Jean became Sean.) in scots, it went to Ian. Joan (irish, again) became Soibhan, (she vawn). it went to Ivan in some russian languages.

for alexandria, there is Alex, alexis, and sandy and lexy..

and i don't know which name forms the most nick names, but i'd vote for margaret
Marge
Margy
Meg
Maggy

Peg
Peggy
Peggeen, (as in dickens)
as well as informal ,Mags, and Mar.