If your receiver or processor only lets you have 1 crossover setting for all speakers, then consider getting surrounds with a frequency response similar to the mains and center. If the -3dB point for your center and mains is, say 50Hz, and the -3dB point for the surrounds is 100Hz, with the crossover set at 80Hz, you'd have a hole in the rear sound field. Also consider whether you prefer dipoles or monopoles, if the speakers are front or rear ported if they need to be wall mounted, and although this is more important on the fronts and centers, I'd get them from the same manufacturer as the fronts and center so they'd be timbre matched. I'm sure more will chime in with further suggestions, but that's what I'd start with.