This is a really old thread, so not sure many people will even notice your post. Luckily AVS notified me because I posted the last one.
Anyway, you may have already bought your speakers by now so this may be redundant. But for what it's worth...
I don't have any experience with in-wall speakers, but I can assume the ones you are talking about are front firing. For the surround back and sides, it is much better to use dipoles if you have the room. I use the Paradigm adp-590's for that very reason and the four of those in the back are just incredible. The surround soundstage behind and to the sides is just superb. And since you mount these on the walls anyway, they would not take up much more space than the in-walls would.
For the fronts, the Studio 60's and 100's excellent speakers (I have both, for front main and front wide), and yes you can buy them all separately, but I would not recommend using another 60 for the center channel. That speaker is not meant for side lying and the accoustics would be off. You are much better going with a properly designed center channel speaker. I have the Paradigm cc-690 (you can see it in my pictures a few posts back, sitting on the shelf above the monitor) and it is very powerful and provides the perfect match for the rest of my setup and delivers a superb center soundstage and excellent dialog.
I also run a Paradigm Servo-15 sub which is 1,500 watts continuous and 4,500 watts peak, so it is more than powerful enough for me and provides incredible bass response down to 12 hz. But now Paradigm has a better one (the signature sub 25) that comes with the PBK (perfect bass kit) which allows for perfect setup using the supplied mic and some software you install on your PC. After listening to the test signals, the PC generates the correct settings and you upload those directly to the sub to configure it. It's a great idea and is a bit like the Audyssey setup in most AVR's these days. I used Audyssey to configure all my speakers of course and it did a great job.
Anyway, I hope you end up with the best setup that works for your budget and your room... and when it's all done, you can sit down in that sweet spot and enjoy every moment of it.