Okay sorry I've been busy today. I know that if you need to use all in ceilings there's a way to do it for the surrounds and rear surrounds you just need to Google it it will show you where to place them
Your going to upgrade the system when the kids are older so you got that covered.
What I'm concerned about is subs. Man I'm not trying to bag on them but those new ones are only rated down to 40hz. There are tower speakers that can do that. For that price of 400 you could go with the rsl speedwoofer 10 sub and it'll blow those out of the water. Hell the new polk HT12's are rated flat to 28hz and are at that price. Since price isn't really an issue and you gotta compromise so much with the surrounds you don't want to skip your bass.
HSU Rhythmik SVS all have subs at that price range that would just SMASH those subs. Since you want this for movies and your surrounds are out of position on the ceiling you want to make up for it with quality bass. Duals with any of those companies would just dust those your buying at that price range or close to to that price.
You ideally want a sub that can play flat down to 20hz. And those subs your buying can't go below 40hz. Your going to be missing out on a lot of content.Believe me I know when I went from budget subs to SVS subs it was a whole new world of reality. I never knew what I was missing.
With any of those companies you buy something at that price range set it up near field due to that room size your going to be very happy with what you get. Those subs your buying will leave you wanting for more.
Unless you just want those budget subs until the kids grow up then upgrade later. Anyways thats my 2 cents. Good luck with the build can't wait to see the finished results and your system when you set it up!