Iron/Poison

it either takes place in a "Phantom Lord wins the fairy-phantom war" OR "Phantom Lord loses but immediately becomes a dark guild/Gajeel doesnt join Fairy Tail" timeline, but the former has the most impact on plot potential

either way Jose is cranking up the shadiest business shit like never before, and that kind of blatant mess might catch the eye of say... the Balaam Alliance

and Oracion Seis are kind of the keepers of the lesser dark guilds for the Alliance, because let's face it the underworld in Fiore has a sort of structure (hence why the Alliance exists)

so the Phantoms need to be checked out, so Brain decides arrange some kind of probationary interview/business meeting and Jose accepts bc holy shit that would result in lots of money/power (+bc he's definitely heard of the Alliance as a former Saint, maybe less so the Seis individually considering the Council knew so little about them)

and they need to bring a couple backup enforcers for appearances and show of power

Jose brings Gajeel (primary, obv) and Aria (without knowing who/what to expect having a guy who can temp nullify magic is a good idea)

Brain brings Midnight (primary + similar idea of having someone that can reflect magic etc) and Cobra (obv, there for intel gathering mostly)

Brain flatout tells Cobra before they go thay "[jose] Will likely bring his second in command, who is a dragonslayer, this will also be a good test for you" which just serves to make cobra a lil pissy before the meeting bc ew a 1st gen/old style dragonslayer

and that's how cojeel first meets in the au

I imagine first impressions are

Cobra: "wow this guy has 1 thing on his mind and that's violence, talk about recklessness"

Gajeel: [trying to figure out why all the other dragonslayers he's met so far are so fucking short???] "this guy is like four feet tall i could step on him and it'd be over"