just in case you missed it, there was a recent thread on pros and cons re: matched vs mismatched surround speakers.
Home Theater > 1411326766
Comments in general
Because:
(a) the rears and surrounds are primarily just a "manufactured" and "fill-in" soundtrack without the critical soundstage cohesion and sonic matching expected and required along the L-C-R fronts, and
(b) you are already looking for in-walls with their unavoidable sonic compromises and inherent limitations because of the in-wall build and installation nature in general; and
(c) All in-wall choices are going to be third party OEM brands anyway with a sonic signature different from the 3 fronts anyway , but just a non-critical matter w/o prejudice to te AGON thread highlighted above.
(d) you are going to have to use a lower grade fire-rated in-wall speaker cable anyway (compared to your fronts I'm guessing) for the surrounds and rears, or you will void you house insurance for non-code speaker wires behind the drywall. That also falls in to the overall "compromises" alchemy highlighted herein.
the available fill-in speaker selection choices are large and the system performance ability that will yield your satisfaction is primarily governed only by the time honoured homily that you generally get what you pay for in this hobby.
IN DETAIL:
From personal experiences,
(1) high-end Martin Logan in-walls will do the job admirably.
(2) FWIW, quality-build on-wall mounts can fit the bill with a better performing product that in-walls but are just as easy to install
- e.g high-end Martin Logans (used older model e.g Frescos and Vignettes are sensational...google the reviews [these are hybrid that are very very good] or
- TOTEM TRIBE on-walls