I have to agree with you on principle about the rear channels.
It is a good thing if you can use the same companys products but you certainly don't have to use identical speakers for the rears.
The center channel is the single most important speaker in a home theater system. I am a Dali dealer and I would recommend you take a listen to them before you buy the Sonus. The Dali's are amazing for both music and theater and are a great deal as well, but back on to my point.
I installed a new cabinet in my video showroom, to house a 65inch 1080P DLP rear projector, and I had to modify the cabinet to house the massive Dali Euphonia center channel, but until I modified my cabinet, I had to use a much smaller and cheaper Ikon series unit. The difference in sound when I reinstalled the superior center channel was enormous! The superior center brought the experience to a whole new level of quality and involvement.
So hopefully this will illustrate the importance of the center channel, you want to buy the best center you can and one that will timbre match the left and right speakers you choose.
If you are going the way of doing any kind of real home theater do it right! A phantom center channel is not appropriate for anyone even contemplating purchasing the great equipment you are thinking about.
Also if you are really serious about music I would also go with a really good tube preamplifier to go with the Ayre or any other amplifier you choose. A tube preamplifier with a theater bypass will give you delightful music performance that should amaze you with the right ancillary equipment.
You would use the Meridian for surround sound with the front two channels going through the preamplifier. If you have a good dealer ask to try playing a piece of music through a good tube preamplifier and then to the amplifiers, and then listen to the same piece of music played directly by the Meridian through the same amplifiers, I am willing to bet you will find music played through a really good tube preamplifier to offer a wider soundstage and a delightful warmth and involving quality. If you havent listened to a good tube preamplifier you really should. You may want to listen to an Ayre preamp as well for the same test and see which way you like the sound.
As per the rest of the amplifier channels needing to be the same, they don't. I have over the years sold people tube amplifiers for the front channels with much cheaper solid state amplifiers to drive the center and rears, and both my customers and I couldnt discern a real qualitative difference.
What you are building here is a music and theater system not a dedicated home theater, so unless you are a theater purist you have the flexibility to create some unique solutions.
Listen and let your ears be your guide.