I think matching all the surround speakers is a nice goal. if you can do it But having them all be from the same manufacturer and size/quality is just about as good. If they make a center for a particular series then it's likely well-matched to the others.
Most engineers know people are more likely to have a dedicated center rather than a third to match the L&R and it's still pretty rare to mixes to use the surrounds as fully as the LRC. Atmos is obviously a non starter as the entertaining post about hanging monster B&W speakers from the ceiling noted. But again if you can stick to one brand they will keep you in the tonal zone (mostly).
If you use the system solely for music I guess a full size speaker for a center would be fine although it would be annoying to have to block the UI for the music, unless you can put it behind a screen or use a small monitor. But what about concerts etc where you have a visual element.
I'm very happy with my Martin Logan set up with Montis L&R a Stage center and their on wall aim-able Electrostats for surrounds. I'm cheating a bit with Barefoot monitors for the rear surrounds but I'll get another pair of the on walls shortly. My height speakers are the new ML Sistine in ceiling 3 ways and while they aren't electrostats they seem very well-matched tonally.
The first time I dropped the Tom Sawyer intro on this rig it blew the whole room up. Matter of fact, I think I'll do that again. Bye all.
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