Except for a few instances of intentional paceline practice, all the fast group rides I've been on are more fluid. Sometimes there's a nice single file paceline, and sometimes a few may be riding side-by-side. Some riders rotate through pulling, some hang toward the back and never pull. Some pull only to get passed because stronger riders don't think they're pulling fast enough and go to the front to pull. Sometimes we hit a hill and the strongest riders race each other to the top. Sometimes a rider might shoot of the front and race ahead out of shear exuberance. The thing to do is to go on some rides and see how it works.