For proper signal parsing you should consider those Surrounds, not Rears.
In a 5.1 setup, it is acceptable to push the Surrounds further back. In a 7.1, not having Surrounds but having Rears will throw the sound field off, I would expect. It is important to keep in mind that Rears are not necessarily native to all programming. The Surrounds are!
That said, if you can go with the Red placement and connect the as Surrounds L/R you will have a better 5- channel set up.