That would be too much at this point. While I think there might be more monsters (I believe that was hinted at), the film's focus is going to keep it to the core four (Godzilla, Rodan, Mothra, and Ghidorah). If there are more monsters, highly unlikely Legendary would pay the expenses for miscellaneous Godzilla monsters when they could just make their own versions (say a giant plesiosaur for Titanosaurus or a theropod-looking creature for Gorosaurus or a spider-like creature for Kumonga). Plus, we're not even sure if Legendary is going to renew their Godzilla rights to continue making movies past 2020.
Absolutely! Not only that, this marks the first time in HISTORY we're getting American versions of Rodan, Mothra, and Ghidorah (even Mechagodzilla if you want to count Ready Player One). Honestly, it's super nuts and really exciting.