We just had middle school orientation, too, where we heard very similar things.
About #1: I've always figured the policy of keeping the 6th graders away from 7th and 8th graders had more to do with the parents than the kids. I think for a lot of us parents, middle school started in 7th, so it's hard for us to wrap our heads around the idea of our 5th graders heading off to middle school in three months. Keeping them separated keeps us from freaking out. It doesn't seem to me you can say definitively that there's more difference "hormonally" between a 6th and 7th grader than between a 7th and 8th grader.
About #2: Seems pretty fuzzy to me. Team building happens when there's something real--i.e., academic--to work on. I for one would have been bored out of my skull if I'd had to spend two months on "intangibles" in middle school (YMMV, of course).
Given #s 1 and 2: Grade skip must be looking pretty good right now...
I'll be curious to hear what you decide, and best of luck!
