[@SaturnSenshi]

Well, first things first.  Make it very clear on the themes and tropes you want to try.  A little background or setting description really helps.  Also, pictures and art make it more attracting.  

Now the whole hostage negotiation thing to start with is fine with a smaller group.  As a first timer, I wouldnt have the groups be split up and instead think of a reason how everyone, human and monster would be on the same side at the beginning.

Maybe a mystery group ambushes the exchange, and everyone starts as prisoners.