My room is about 17 by 40. It includes den, study, dining area, and kitchen. I only use the front third for music and HT. Because my couch is low and I have a 26" 20 yr old Sony, that even Katrina could not kill, I use an Academy center. I have K horns up front, paired subs, and can use LaScala's or Heresy's (which currently are right on top of the LS) in the rear. In my room and for my ears, unless you like true 5 ch audio, the LSs in the rear are overkill, so I actually only hook up the Heresy's in the rear. (But the LSs won't fit in the bedroom and I won't sell them). I have never put a LS between the K horns, because if I put my TV on top of it, the TV would be too high for me. I think the sound would likely be grand though. Keep the height of your TV in mind. I also live on the floor for hours at a time. So again, a low TV makes more sense for me. If you have small kids, a high TV may be safe, but they may not be able to see it from the floor.
I agree that for HT the center is very important. I use an Academy. It sounds fine and fits on top of my TV, while only mildly crushing and warping the TV top. Nothing heavier would work for me. Nothing bigger is adequately shielded-although I am told you could use roofers steel from Home Depot to shield the TV if you used Heresys under the TV as a center. That would still raise my TV.
Remeber, your folded horn speakers are directional. Unless you have a projection TV with a semitransparent screen for the horn speakers to shoot thru or unless your TV height is high a single LS or Belle could cause some issues. You want the horns to fire to your ear level. Could the other posters reply to this (unless they just have high TVs).
Just my 2 cents.