Imma trim it down to what's maybe a little more gettable from a numeric standpoint. Also, because I'm running out of suggestions.
Freedom PhalanxStatesman: Nathan Fillion. Done. Perfect for the role.
Citadel: He's going to be all CG, so a good voice actor, and I pick Phil Lamarr. Or maybe Keith David.
Back Alley Brawler: Jason Momoa or Dwayne Johnson.
Manticore: THIS is where you put Diesel. Vin would be awesome at this, watch.
Numina: Some newbie actress, for while important... she's not a main.
Positron: You never see him outside the suit (well, not until the second or third movie, if you go with 'canon'). So he can be Billy West as a character. And then when you do see him in the second movie, you can cast him then, for outside the suit, he can be anyone, but inside? He's Billy West's character, Zac Branagan. Or such.
Sister Psyche: Alice Eve, I think.
Synapse: Brash newbie.
Arachnos:
Lord Recluse: Aaron Eckhardt. Dude could so do this. And if not him? Fine. *Liam Neeson*. NO? Gary Oldman. What? FINE. I don't care anyelse you want here then. *pouts*
Black Scorpion: Jason Momoa or Dwayne Johnson. Whoever didn't get Back Alley Brawler.
Captain Mako: CGI. Pick some good voice actor. Tara Strong could do it, in a pinch, but I'd go with Richard Horvitz or more probably Jess Harnell.
Dr. Aeon: This needs to be NPH. NEEDS.
Ghost Widow: Charlize Theron. I'm tellin' you.
Past those, get whoever ya want.