The “sillier” tone will be harder to pull off if Deadpool isn’t part of the team, but like Reese said, the goal with X-Force isn’t to merely be another cog in a giant machine.
Considering how involved Rhett Reese and Paul Wernick are in getting the X-Force set up in this universe, it’s not too surprising that they want it to resemble Deadpool more than the X-Men installments.
Plus, in the comics, the X-Force is a blacks ops team of mutants that is more willing to take brutal action against any enemies against mutantkind.
They want the militaristic group to help expand the X-Men universe, but they also want them to lead an enjoyable, self-contained adventure; one that’s packed with plenty of action, but is also self-referential and humorous at times.