To the OP:
Based on looking at your sketch, it seems our rooms are very similar -- almost frighteningly so! I also have in-ceiling surrounds, as they were there when we bought the house (considered an upgrade in our neck of the real estate woods; see my signature). There's no problem with the distance or positioning, as 'Rat said, but you just need to adjust everything in your AVR or processor to compensate...in other words, make sure your distances are correctly set as well as the channel level balance so one surround doesn't sound "hotter" than the other, etc.
EDIT: I see you decided to perhaps go with bookshelves or satellites for the surrounds; if you do go the in-ceiling route, I'd suggest looking at SpeakerCrafts, as are in my ceiling -- I was advised these are pretty premium as far as in-wall/in-ceiling loudspeakers go.
Don't do the behind-couch bookshelves on stands; I went down this route in our last apartment and they were constantly getting knocked into and all other kinds of stuff -- especially if they border an area that's constantly being walked past.