TVLine reports that Girder will be introduced in episode six of the Flash TV series. The casting call notes that he was a former bully of Barry in childhood, and can “transmute any part of his body into solid steel”. So he’ll obviously be a bit different from the character in the comics, but is keeping some of the basic elements. He has not been cast yet.
Firestorm Fan reports that Kelly Frye has been cast as Plastique. She’ll apparently debut in the fifth episode, and will also be a metahuman due to the same accident which gave Barry his powers. Frye has appeared in such dramas as The Mentalist and NCIS: Los Angeles.
Geoff Johns gave a fairly lengthy interview to CBR about the Flash TV series and some of DC’s other television properties. (There are a few spoilers for the pilot episode referenced in the interview.) Johns talks about the Rogues and other villains they might be using, and discusses later plot points which are seeded in the pilot. And there’s a fair bit about other TV series, if you’re interested in that.
CBR also interviewed executive producers Andrew Kreisberg and Greg Berlanti about the tone of the series, and synchronicity with shows like Arrow.