We were allowed in at 4pm on 10/01 this year with no problems, and used no other park passes. The official Disney line states 6:30 - but I can tell you, it's 4pm.
That said - the rides were walk on, but the crowds were MUCH worse just walking around the park. We had a great time - but I don't know if it was worth the extra park tickets. I would have rather just gone during the day without so many people! Our kids had a great time dressing up and getting candy, so it was worth it for them. The fireworks are pretty impressive, and the decor is rather neat.
If it's the only chance you'll have to go to MK - I'd say go on a regular day, and skip the party. Have fun!