1) I'm not familiar with either speaker, but use your best pair for your main left and right speakers.
2) You can use any speakers you want for the rears. Usually, in order to mount them on the wall and out of the way, people use bookshelf speakers for the rears. I would let the physical size of your speakers be the determining factor here (smallest pair gets the job).
3) What is your price range? NHT makes a pretty decent center channel called the AudioCenter 2 that is about $800 new (I'd look for a used one for around 50% of retail).
4) You don't have to have a sub if your processor allows you to route the sub's low frequency info to the main speakers. But if your main speakers aren't full range, your system will be seriously lacking down under (ok, bad pun). Even if they are full range, there's no substitute for a good sub in your system if your neighbors will tolerate it.
5) You'll have to A/B test it to see how much of a difference it makes. Having the NAD in the chain is less than ideal, but routing the main speaker info through the NAD's preamp outputs is your only option unless you want to physically unplug the main left and right inputs from the receiver and plug them into your S200 each time.