Yeahhh ... I'm trying to figure out what your fi's rush is.
This is a relationship building, right? You don't show up for a first date with a girl with a ring in your pocket ready to propose under "normal" circumstances. Why would you do it when pursuing a poly relationship?
SEems like your fiance thinks that this is just a done deal? I think you should talk to him and ask him why he wants to move so fast. See what his thoughts are behind trying to create an instant relationship.
I wouldn't proceed with it until you know what's going on in his mind.