My room is about the same size as yours Patrique - 12 x 18. I have rear surrounds, and would recommend the rear surrounds to anyone. I am also using M60's, a VP160, 2 EP500's and I also have height and width channels in this room. The width channels are hit and miss and totally dependent on the source material. I rarely use them. I do however like the height channels and use DD PLZ primarily. I use QS8's for the surrounds and height channels - so I have six of them in this little room.
Having experimented with different speakers and amps in my room quite a bit, if you have the means, I suggest you go 7.1 with a EP 500, M60's, a VP160 and QS-8's. It will not be too much.