Well, Shikamaru was created by Masashi Kishimoto in the late 1990's/early 2000's, and Fairy Tail as a whole was first published in 2006. The characters are very similar, but I wouldn't go out there and call it 'copying' or 'plagiarism' or whatever. *_*
Maybe its a way that Hiro chose to honor an accomplished mangaka like Kishimoto by creating a character that is a basic copy of one of Kishi's?
I mean, that's the best reason I can think of. The characters look the same, kinda sound the same and have the same kind of power over shadows? It's got to be more than a coincidence.
If it was a blatant copy, Kishi would have sued him or sumthin'...but nada...so this seems like the most probable thing.