I wanted to know why their emblems are on their shoulder. I realized it days ago. If any of you have any answers or theories, please tell me. I would appreciate it a lot.
@Jerry is right, the placement of the guild's symbol is wherever the mage wants it. If you remember in Volume 1 Chapter 2, Mirajane was asking Lucy if she was sure she wants the symbol placed on her hand, implying that mages have a choice as to where they can put it.
As to how some dragon slayers have their guild's symbol in their shoulders, that is mostly coincidental and if there was any special meaning to it, it was not focused upon nor explained throughout the series, at least in the manga, if I recall correctly.