Consider this all rumor for now. If it all pans out they'll be joining Robert Downey Jr. as the only confirmed actor for the Joss Whedon directed film.
Fillion is just geeky enough to get the Hank Pym character just right. The surprise is Longoria, who is the last person I expected to see involved. Dammit what happened to Oliva Munn? I'd much rather have her, bad acting and all.