Are you having an Escape wedding? If you're having a Wishes I don't think there would be any issues. I personally had no issues at all with having higher guest counts at everything except the ceremony, but there are reports of others having problems. I was very clear when we started our planning that we did not want a Wishes wedding because we had no way of filling a room block since many guests were locals.
We haven't addressed your Fantasmic DP questions yet! There aren't a lot of these, but I know one of the DIS brides had one. I believe it was Mary_626? Forgive me for not remembering the screen name! Maybe someone else will...
From what I remember from her TR, the Fantasmic! DP is held in a small patio area at the top of the Fantastmic! "stadium". It seemed to be just like the Illuminations DP as far as food is concerned, but maybe not. I'm not sure what the rules are at MGM as far as formal (wedding) attire is concerned. We used Disney transportation to take us backstage so we could be at our DP in our wedding clothes.