The back surround location shown will work, but it'd be better to increase the separation between the side and back surrounds by moving the sides up a foot or two and rears back to the extent possible.
The speakers that you'll use are a bit smaller than usually suggested, but with a good sub handling the bass, should give an enjoyable effect. Keep in mind that your room doesn't have to be "filled" with sound at a high level. The requirement is just to have an adequate level at your listening position.
