I think that by age 6, a drop-off party is assumed. You could specify "siblings welcome." I've heard that some people expect siblings to be invited too, but that is a totally foreign concept to me! It's your son's party for him and his friends.
Toy Story 3 just came out on video, so maybe that would be a good choice for a movie. I think a movie like that is generally unoffensive, so I wouldn't feel the need to approve it with parents. If it were something more mature, say Harry Potter or Spiderman, I would call the parents personally and ask if that's okay for their child. They can be too scary and mature for kids this age, and it's a personal choice that parents make for their kids. But like I said, a Disney animated movie is probably safe.
I think pizza, soda, and cake will be awesome. And I don't know why you're worried about sugar before bedtime. Isn't that what dessert usually is? While "sugar rush" is a common saying, it's not true. At least for most kids.