In a narrow room like that, I think it would make a lot more sense to use the in-ceiling speakers as the side surrounds and the in-walls as rears. If you place the sides where they are usually recommended to go, you will have some listeners sitting right next to a speaker, and that would be distracting. Also, if the in-ceiling speakers can not be aimed, they will either be projecting sound straight down onto no one, or down right on top of the listening position which probably won't sound like what the rears surrounds are supposed to.
If you just can not use the in-walls for the rear-surrounds, I would place them high up where they won't be directly into someones ear. If you have to use the in-ceilings a rears, I would place them just a bit behind the rear seats, maybe a foot or two. If you place them too far back, they won't be heard at all. It's not optimal, but it could still sound fine. Here is a diagram which can be used as a general guideline for 7.1 placement:
Definitely don't use this diagram as a hard rule for subwoofer placement.