Speakers A is for your main area/zone. Speakers B would usually be for an alternative area. The Amp assign on the advanced menu, may allow you to change this, not to sure on the Onkyo.
Usually when you can listen to two different sources in two different rooms, it would be referred to as Zones 1 & 2 (&3... etc)
When you have to listen to the same thing, but can switch on and off independently, it would be referred to A & B.
Speakers for rears are less imperative to match with your front speakers ito brand but basically make sure that they are in accordance to the power output of your amp. Generally a basic 4-6" within your budget will work the charm, as long as they do not overpower your front speakers, which should be the bigger speakers in your system (So yes, B&W or wharfdale will do)
A Power amp is standalone amplification (without processing). Imagine taking your AVR and dividing it into two pieces... one for decoding ( the processor, where you plug all your sources into) and one just for power (the poweramp - where you would plug your speakers into) and connecting the two to work as one.
Hope it helps!