Do not mix center andL/R.  You can get away with mixing the rear speakers if they are not too different (but how can you know?).  I would stick with all KEFs or all Polks.

I think that the KEF speakers are a very good choice.

The C7 will be a good match for the Q200c center speaker and the C1 rear speakers.

There are several good receivers available, but I would advise you to NOT get Yamaha or Sony, due to poor sound quality.

The ones I would suggest are the Cambridge 551R or 651R, the Denon 3312, the NAD T757, and the Marantz SR6008. You have a large room, so don't get an inadequate receiver.

Since you are going to do mostly music listening, the Cambridge would definitely be the best if you can find a dealer for it.