Greg responds...

I believe it went down like this:

We needed to cast Elasti-Girl for "Nightmare Monkeys". As that episode included "Doom Patrol Go!" as a tribute to "Teen Titans Go!" we decided to use the TTG voice cast for the DPG characters, paralleling each as much as possible.

So...

Greg Cipes = Beast Boy = Beast Boy
Tara Strong = Raven = Negative Woman
Khary Payton = Cyborg = Robotman
Scott Menville = Robin = Chief
Hynden Walch = Starfire = Elasti-Girl

Since Hynden was now the official voice of Elasti-Girl, that meant she was now also the official voice of Rita Farr, who became Elasti-Girl, as well as the official voice of Rita Lee, the character that Rita Farr played on "Hello, Megan!"

And since Em'ree was ultimately going to take on the image of Rita Lee, Hynden was the only choice to play Em'ree.

Of course, the fact that Hynden is amazing didn't hurt.

Make sense?