Could always go with MarvelMAX Deadpool to be honest, just completely insane but doesn't break the fourth wall as much.
As for the other's, I'm not sure about Cable being the lead characters, I always find him a struggle to read in comic book form; only Cable & Deadpool is the only Cable comic I've managed to buy more than the first collected of; the characters make up for it if they did go with the X-Force movie.
I will be starting Deadpool Max once I have picked up Second Coming: Revelations, and The Punisher: Franken Castle to finish those runs. Being an Ennis fan I have only read Punisher & Fury Max. But, one of my favorite things about Deadpool is him talking about what should be an inner monologue, and the psychics around him thinking he is completely nuts to believe he is a comic book character.
For me Cable has always been very different from the rest of Marvel's Alpha level characters. Most importantly he is not an Alpha Female(Jean Grey, Rachel Summers, Hope Summers, Wanda Maximoff, & Sue Richards, could all kill Hulk or Wolverine, nevermind characters as powerful as Thanos or Galactus), until recently he had all that power, and had to use it just to keep his body from falling apart thanks to the Techno-Organic Virus(for those that don't know, its apparent in Cables victory celebration in Marvel vs. Capcom 2) because Apocalypse himself feared the overall power Cable possessed. Its kind of like Superman learning how to be a superhero while being encased in Kryptonite. And being a soldier sent from the current timeline into the future by his parents (Scott Summers & Jean Grey) to protect him, then having to return to our timeline to save the future makes him like a badass Booster Gold(and Amalgam made a huge mistake by not making Booster Cable). I can understand people not finding him to be a great character, but with X-Force they have a chance to do a very different kind of story, and have the ability to be slightly darker, having Wolverine as a secondary character would still get casuals to watch, including Dom, Wolfsbane, or X-23 gets the sex appeal, and Cable would be a chance to establish a veteran of wars, that if he is successful will never happen(which could actually create the AoA without the use of David Haller)
Psylock concerns me slightly, as you need an Asian actress who can pull off decent stunts, with either a British accent, or voice over. It just seems like too much work to put into a character in film.