Well Harkon already feela kinda like the wannabe "Dracula" of TES (at least ti me he did).
As for siding with Werewolves...ugh no. I love Werewolves as much as the next guy but as stated in many similar topics: Werewolves and Vampires in TES are not natural enemies. Sure there is a Vampire that mentions Werewolf blood being delicious and Harkon shows some distaste at you being a Werewolf, referring to it as an "impurity", but that is just him being a snob.
As much as I love Underworld, I think Dawnguard got more than enough influnce from there already. It really dorsn't need any more.