No. the sixth channel in a 6.1 set-up is not a discrete channel--it is just the two rear channels added together, thus it really adds nothing, particularly given the dimensions of your room. 5.1 is the standard and everything is mixed for 5.1, so I would stick with 5.1 and not worry about it. In fact, you would hear more and better sound buying five higher quality speakers than 6 speakers of lesser quality.
Actually the 6th channel is discrete but there are not to many DVD's out their that have the discrete ES channel mixed in. Rush Hour 2 is an example of a true 6.1 movie. I run a 7.1 channel set-up and love it, the back 2 channels are matrixed in a 7.1.
I'd recommend buying a 5.1 speaker setup and a 6.1 amp and you can always buy 1 more speaker later. Remember, the rear speakers are more fill and effect than anything. I have a room about your size and 5.1 does awesome.
I have a 600watt,7piece home theater audio system w/3-dvd/cd changer.If I take away 5 of theses speakers,Can I hook up a 400watt speakers that carry 12" woofers/4" Midrange/and a high SPL Professional tweeter horns(3-way P.A./D.J./Nightclub speakers).Please let me know before I purchase....Thank You !!!!!!