The MouseFest folks will be there, most likely, on the 11th. But, they aren't a huge presence. Yes, there may be a few hundred of them, but unless you are in the same area they're in (usually down in the Rose Garden) you most likely won't even notice they're there.
This is a very popular time of year for people to go to WDW. POP Warner is just finishing up their week in WDW, so a few may go to the party on either night. There are a lot of DVCers who go at this time of year as well. I have been to the parties that Thursday night many times and it has not been horribly crowded. This year I'll be there on Sunday night, for a change.