There's no reason you have to use models for what they were intended - apart from Brother Peter, the Armed Brothers will work very well as a monastic order of Vampire Thralls in a Nosferatu Gang with the addition of a Vampire or two.
The only thing I would say is that you probably need a couple more miniatures to make the Gentlemen with Guns work as a faction.
Personally, in your position, I'd consider the Vampire Slayers. I know it's not clear what they are, but there's a good chance that either or both of the Fallen Woman and the Lost Love will be vampiric, and the other miniatures in the set would fill out your Gentlemen's Club.
Having said that, I also think that a good option is to simply choose the models you like the look of and get creative in how you fit them together when you have the rules in front of you.