What Revel rear speakers to use?

I would be less concerned about the rear channels than the mismatch between your fronts and center channels. The "logical" change would be to replace the C-30 with either another Gem or a Voice from the same top tier Ultima line. The C-30 while nice, is from the entry level Concerta line. As the center channel is arguably the most important-and should tonally blend seamlessly with the fronts, this is the place to start IMHO.

I agree with Spinaker in that the seamless intregration across the front 3 speakers is a priority in HT setup. However the C30 is from the predessor to the Performa F32/C32/M22 line up. The C12 is from the Concerta line up.
To find the rears that match with the C30 and F30 I believe that those were the S30 which were a di-pole design.

I bought a used pair of the original Revel Studios last year and pretty much went through the same search as the OP. After researching it to death and given my budget, I ended up agreeing with Spinaker that the surrounds were not as critical. I got a pair of S30s and am very happy with the results. They can be switched between dipole and monopole to adjust for music vs movies. The only caveat is that they are on-wall speaks and that they do stick out a bit so you should make sure you're ok with that.