> Stevie is Stephanie which has always been very much a girl's name. Abbreviations have a tendency to collide.

Maybe, I guess. That would be one of those "abbreviations" like "Peggy" ("abbreviating" Margaret) that doesn't actually abbreviate the name, and you just have to know that someone out there thinks it's related.

"Stevie" for "Steven" follows the ordinary rules of English. For "Stephanie", you'd expect "Steffie".