Notably, the reconstruction of the Two Medicine formation would largely have creatures from that formation, rather than just creatures from Hell Creek apart from Gorgosaurus and Styracosaurus. The Two Medicine formation was, however, simillar to Hell Creek, in terms of superficially simillar fauna. There were ankylosaurs, both nodosaurids and ankylosaurids. There were avisaurids. There were "parksosaurs", caenagnathids, tyrannosaurines, etc. There was a much bigger abundance on centrosaurine ceratopsians, such as Achelousaurus and Einiosaurus, which some believe to have been gradual descendants of Styracosaurus. There were leptoceratopsians, such as Cerasinops and Prenoceratops. Deinonychosaurs were represented by troodontids and dromaeosaurs such as dromaeosaurines and Saurornitholestes. There were plenty of hadrosaurs, including the lambeosaurine variety, characterized by "Hypacrosaurus" stebingeri, and saurolophines such as Acristavus, Gryposaurus, Maiasaura and Prosaurolophus. Pterosaurs were represented by a weird variety, with the small indeterminate Piksi, the mid-sized azhdarchid or a relative of azhdarchids, the Montanazhdarcho, and a giant azhdarchid, possibly Cryodrakon. Mammals are also known, such as the multituberculate Cimexomys. Lizards such as Magnuviator were also present. Bivalves, gastropods, turtles, champsosaurs and insects were also present.
It would be a rich ecosystem, of course, but I'm afraid it would have been a little too much ambition for the devs, so for once, I'm pleased they didn't met the goal.
As for different playability that Gorgosaurus and Styracosaurus might have... I would suspect they wouldn't make them too different from either Tyrannosaurus or Triceratops really. Either it be for the sake of not giving too much work on making their playability options too different, or because there's not too much evidence for a radically different lifestyle for the creatures.