Wow, I just checked out the Aquatic Gardener's Assoc. site - some of those contest winners are amazing!
I'm totally new to plants, but as far as I'm aware, trimmed stems are desirable in some places - when you take a top cutting, the bottom stem will branch out to the sides, creating a 'bushier', fuller plant. It might look a bit raw at first, but I'm sure after a week you don't even notice that it's been trimmed.
I don't know if you want to put cabomba in the front - it's insanely fast growing, so you'll be constantly pruning it. I use it in the back of the tank to hide the filter uplifts and heater, but personally I wouldn't have the energy to trim it ever few days if it was in the front.
That space near the centre of your tank is where I was thinking a specimen plant might look nice - there are a lot of swords/crypts to choose from. For a 20g tall, I'd pick something that isn't going to get too large, you don't want one of those swords that get 24" high with an 18" spread
I like your drawing, I thought just for kicks I'd show you what I meant about groupings of the same plants, so I made a drawing of my tank's layout - I've never tried attaching anything, hope this works!
