The problem with this is that Disney's system doesn't know that you are trying to use the FP system just for the party, or for the whole day. I, and MANY others, have had issues with that. For example... I have a 4 day ticket that I plan to use Mon, Tues, Weds, (NOT Thurs) and Fri and for Thurs I will use my MNSSHP to get in to park.. If I try and make fast passes Mon-Thurs, the system thinks I'm trying to use my 4th day park ticket for Thursday, so it wouldn't let me make FP for Friday (even though that's going to be my 4th ticketed day.)
Make sense?
In the past when this happens, I've had to call and sit on hold forever and talk to a couple of CMs and they basically give me "complimentary fast passes" since they understand what's going on, but there's nothing they can do about it in the system.