I can only tell you of my experience.
My room seats 8 - in two rows. 4 in front row, 4 in rear. The second row is raised about 8".
Total room length is 21' width is 12'. So to answer your square vs. rectangle question, mine is a rectangle and it works great. Great sound too.
A few other things:
1) I would check out other projectors over the Panny. If you read other sites like ProjectorrCentral, they are far too pro-Panasonic. You'd be far better to pick up a JVC-RS4810. It has lens memory like your Panny, and it is a much better PJ. But don't take my word....comb this forum and you'll find better options, I think.
2) if you're looking to save a few bucks, I would consider a 5.1 surround system. I've been to several friend's homes with 7.1 and quite honestly, I've never noticed a difference from my 5.1 set-up
3) Best piece of advice I could offer - run conduit from your components to PJ. I just ran cables, and had to smash through drywall when I needed to change a cable. Don't make that mistake.
4) you're going to want to paint your ceilings and walls as dark as possible. Black is ideal.
Just my 2 cents.