I guess I don't remember reading any specifics about the "real" HMS Ontario being introduced into the game, but I'll take your word for it. The Pirate Frigate, as far as I know, is a just a variant of the regular frigate. I don't know how different its stats are. The "generic" 74/3rd Rate uses one of the Bellona's old models/textures (without the Bellona's ornate stern), but might have a different armament range. I'm sure there are other forum members who know more.
Also, there's this ship: http://forum.game-labs.net/index.php?/topic/7165-what-is-the-ship-on-this-screen/ Gros Ventre, a light merchant ship.