You say rears, I assume your meaning left/right surrounds? If so, you would normally want them to the left/right of the viewers approx. 20degrees in front or behind. For the rear channel (7.1) it is ok to use bookshelves for the rears, but I would go with Q's for the left/right as the experience would be much better.

You have no way to put some stands to the left/right side for Q's?

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85