I had been told by the owner of my local store that the Dark Magician Girl's 200 Atk bonus applied not only to the amount of Dark Magicians (Atk2500/Def2100) in the graveyard, but also to Skilled Dark Magicians,Dark Magicians of Chaos and even other Dark Magician Girls. Is this true? Or should I hunt the store owner down with the help of a lynch mob and treat the town to some good old fashioned vigilante justice?

Nope, it only applies to Dark Magicians and Magician of Black Chaos (WHICH IS NOT the same as Dark Magician of Chaos)

MoBC is a ritual monster.

DMoC is not.

this guy is right, it only works for dm and magician of black chaos, not for dark magician of chaos and sdm

The name must match up EXACTLY as "Dark Magician" or "Magician of Black Chaos." It cannot be "Skilled Dark Magician." It cannot be "Dark Magician of Chaos." It must be EXACTLY "Dark Magician" or "Magician of Black Chaos." I don't care how similar any of the cards look to Dark Magician and Magician of Black Chaos. If their EXACT name is not "Dark Magician" or "Magician of Black Chaos" then there is 0 ATK boost from those monsters for Dark Magician Girl.

The names must be "Dark Magician" or "Magician of Black Chaos."

It DOES NOT say "Increase the ATK of this card by 300 for every card with 'Dark Magician' in its name in either player's graveyard." <-- if it said this, they would work, but it does not, it specifically designates Dark Magician and Magician of Black Chaos (the ritual, which is japanese).