One of the things that stumps me as far as rounding out the full cast, is that Harry and Thomas would have to resemble one another close enough to pull off the brother-act. If you go the Padalecki route, you could cast Taylor Kitsch as Thomas, who is basically a prettier Padalecki. And you have two actors young enough to build a franchise around. There's a height difference, as apparently Pad's 6'4" or more whereas Kitsch is 5'11", but if I remember correctly Thomas is shorter anyway, though I could be wrong (Something that occurs frequently when I argue with my wife).
You are correct, Thomas is shorter than Harry, and Harry frequently lords it over him. (Plus, there was that moment in Small Favor where Thomas had to run around town as Harry to distract the gruffs, but the glamour Harry had created for him reflected the way he [Harry] looked before he got his nose broken by a gruff's hoof. They had to get Molly to use her Goth-'Em-Up make-up to make Thomas actually look like Harry, and she had to practice looking past the glamour, since Thomas's eye line was notably lower than Harry's was.)
I think most people who root for Padalecki to play Harry would prefer it if Jensen Ackles played Thomas, because in real life, as well as on
Supernatural there is a four to five year age difference between them, Ackles is the older, shorter, "prettier" one (some would say), and the boys have a history with each other, both as playing brothers, and being as close AS brothers (Ackles was best man at Padalecki's wedding to Genevieve Cortese; prior to that, they were actually roomies -- I believe -- since 2006.)