Within the available used SNELL model variants -- especially for a 5.1 setup wherein the CC and RL and RR are a non-critical manufactured audio track-- , it won't matter to have that identical variant to your main fronts. To identically match them all is a nice-to-have, but not a must need -to-have choice.
its just more if a speaker appearance matching thing ..... speaker (veneer colour and finish .... IF that is even a hyper-critical issue at all for your 5.1 speaker expansion plan.
in any case, even a slightly different newer/ older variant of the same SNELL model speaker to serve as your CC, will be a quantum leap closer (if not even identical) then any 3rd party speaker choice, to matching the bespoke sonic signature of your current fronts and blend seamlessly with the fronts , .... full stop.
because the rears in a new 5.1 rig are just a pure fill-in manufactured sound track , it won't matter by comparison to try a clone to fronts.
IMO just go snag a couple of used pairs to flesh out your multi-channel speaker expansion plan.