Not sure if they ate Matoran,but it sure would be interesting.Pridak did try to bite Dekar in the movie,so...
If you mean,turning into ''shark mode'',then i guess not.I'm thinking that the model simply has two feet because it would else be very unstable, but that in the story,Pridak constantly swims like in the movie.
Yes,it's a 300-foot long eel, that attacked the tunnel that the Toa Inika were in when they were on their way to Mahri Nui.The tunnel is the ''cord of stone'', by the way.
I'm guessing that the creature destroyed the cord, thus exposing the Toa Inika to the mutagen water,thus turning them into the Toa Mahri.