We don't know what the tone of X-Force will be. Being a sequel to or offshoot of Deadpool 2 doesn't/wouldn't require "aping" the tone of that film.
Yeah, but Deadpool isn't going to be in a completely serious film. Sure, I doubt characters like cable will be breaking the fourth wall, but its still going to be a fairly humorous film if it includes Deadpool, much more then the average X-film.