In Fairy Tail, they say that Dragon Slayers are really rare, but it seems like there's more of them than most other kinds of magic. The closest amount of one kind of magic is maker magic; there are 5 people who are shown using it (Lyon, Gray, Ur, Ultear, and Rufus). It's kinda the same thing with Celestial Wizards, they say there aren't many of them but there are 4 in the show (Lucy, Angel, Yukino, and Hisui. There's also Karen who died 3 years ago and I think there's more in the anime during the Clock Arc, but I don't think that is in the manga).
I kind of get why the Celestial Wizards were likely to interact because they were needed for stuff, but if there aren't many Dragon Slayers, wouldn't it make more sense for them to run into more of any other kind of wizard than for 7 Dragon Slayers to just happen to find each other? Other than when they run into Leon, they never meet anyone with the same magic as anyone else in the guild except Celestial Wizards and Dragon Slayers.