Hi NathanE,
Depends on the size of the room to be honest with regards to speakers, you can either go for sub/sat in a smaller room or larger full size loudspeakers if you have a large room.
It's not essential to have a centre rear but does make a differnce by filling in the hole between the two rear speakers, if you watch more films than music go for the centre rears if you can, if you listen to music more keep to a 5.1 system and use the Denons extra 2 channels from it's 7 to bi-amp the front left and right to improve the 'stereo only' sound.
You can use any speakers as rears, just remember to try and keeps each speaker the same make/model if you can to keep the sound 'uniform'.
Hope this helps, regards, Xan.