This particular rumor comes from the RPF (Replica Prop)forums by a user going by the name of Jet Beetle. He claims to know some producers over at Warner Brothers who have leaked some info to him.
"A couple of producers I know told me they were involved with the upcoming Justice League movie - I've known these guys for years and as nice as they are they have never been involved with something as big as JL, so who knows. They did say Warner is basically trying to follow Marvel's lead (no surprise there) and amongst the heavy hitters already expected to be on hand (Superman, Batman, Wonder Woman, Aquaman) they want to put in a comic relief character (they seem to believe this is what Ant Man will be - no matter how much I defended the character I had to admit I had no idea how Pym will be handled in this upcoming film). Then when they told me the character Warner was planning to include.... well, I smiled and could totally see it as a good thing. The idea of Plastic Man bouncing all over the place and the fact that the studio is looking at David Tennent to bring him to life -- it was the first time I got excited to see a Justice League movie. So let's look at the characters - and seeing how none of us know what the line-up will be, go ahead and put in your personal fantasy players and who will play them."