Re: Billing order.
I think its safe to say that the 14 actors who attended the Comic-Con are the principal/main cast.
So they are the ones who would will get a billing in the posters.
X-MEN: FEMME FATALE
Storm | Phoenix | Rogue | Psylocke | Polaris | Dazzler
Emma Sage Kitty Jubilee M Husk Marrow Cecilia Blink Lifeguard Stacy