How wide is that back wall? Can't really tell because there is no scale on the drawing.
I would say though that if you have room for one speaker, you have room for two. They can both be placed right next to each other, about 1 foot apart. That is actually what THX recommends.
You probably would be better off with monopole speakers because there isn't a wall surface to the left and right to bounce sound off for a di/bipole. Some people consider 6.1 to be a flawed format because sometimes with a single speaker directly behind you, you get a psychoacoustic phenomenon known as 'back to front reversal' which is where your brain perceives the sound that is actually coming from directly behind you to be coming from the front. Not everyone notices it and it won't occur on all material (I have no personal experience with it), but its something to consider.