Pauly is a professional like sniper. That's Why i was saying luna, she is more professional. And trixie was going good to be merasmus. trixie often says: "the great and powerful trixie" (she is an exibitionist) and merasumus: "cower you fools! Merasumus is here!" (He is also an exibitionist. as for discord, i would like to agree, but... the director likes to keep the mercenaries in line. And who is better in that than the ruler of equestria? For the choice of saxton hale, i like it. and, as for red and blue mann... well let's just say this: they are (in the comic) a pair of j**ks. The flin-flan bros could do good. or someone else....
Thing is Luna is not the 'reserved bookish type' that Pauline rubs off as - Twilight'd be perfect for her if it wasn't for the 'being a playable character' deal. Luna is blunt and often unsympathetic - she'd be more likely to be the announcer than Pauline, and due to her duality with Celestia, it would not work.
Trixie and Merasmus are different - Trixie is a performer, and Merasmus is just a ham. Merasmus is also more powerful than the mercs - by default, this'd make Trixie more powerful than Twilight, thus we have a deal breaker. Luna might work, or Sombra, or Star Swirl - but not likely Trixie.
The Director doesn't 'like' to keep the mercs in line - she serves a function. That function is as of yet unknown, but the recent comics may yet shed light on it. Discord has a purpose too - his own amusement. I could easily imagine him wanting teams to fight just because there was nothing better on TV at the moment. Luna and Celestia would make better choices for Redmond and Blutarch if we're going by 'ruler' standards, because as said, the Announcer is little more than a middle woman.