Some questions you have to ask yourself:
1. Are you going to pay for all the rentals and masks for people to wear? I know this is you and your fiance's day and all, but I would think that some guests might not be down to wear costumes/big ball gowns and wear masks all night. Instead, have them come in their formal clothes like normal, you and your fiance can do a change out into something else, if you wish, and have masks on the table as card holders or as part of the centerpiece.
It's not that it would difficult; people would get it, trust me. Masquerade is pretty obvious and no one will think it's something that doesn't go with your ceremony. You can make the transition.
Edit: If I were a guest at your Victorian wedding, I wouldn't think twice about your reception being masquerade. It was a very popular thing to have in those days, and Halloween/Fall, where it may have been the preferred setting for such a party, didn't even come into play, as these parties and balls were help year round for the wealthy and middle class, and yes, even the poor got down having their own good time. The Victorian era up until about the early 1920's is when excessive and formal masquerade balls were seen as passe and were pretty much done and over with.
One of the great things about masquerade is that it's pretty much the ONLY party/theme/dressy event that has stood the test of time and can still be seen from time to time. Sure, it has its novelty now, and they aren't taken as seriously as they were back then, but they are still alive and well with conventions, annual gatherings for a huge fan base, and still a go-to when you can't think of something to jazz up a run-of-the-mill Halloween party or wedding reception.
You are keeping with the tradition of masquerade, which was a simple elegant costume to perhaps slightly make a point of your overall idea of who/what you are supposed to represent, but it's all about the mask. Nowadays, they make both the dress AND mask the whole deal, when back in the old days of masquerade, it was the mask that was heavily detailed and supposed to be the pop and pride of the whole thing, not the dress.
IT WILL NOT CLASH AT ALL :)