Yes, they are in one set of costumes at the pre-party, then another set during the party. Last year they were on the trail to Big Thunder Ranch. I know that area is closed now, but not sure exactly where so have no idea if they will still be in that area or somewhere else.
We were somewhat early in line for the pre-party and knew where to line up for Mickey and Minnie ahead of time, so were first to meet them, so very little hassle. We didn't get to them during the party until about an hour into it (since we met Villains first) and their line was crazy long and crazy slow.
Photopass is out during the party, but was not out for the pre-party. They did have character handlers who would take pictures on your own camera though.
This was the pre-party last year:
Toontown Mickey & Minnie by
mom2rtk, on Flickr
And during the
Mickey and MInne at Mickey's Halloween Party by
mom2rtk, on Flickr