<font color='#000000'>Apart from possible timbre mismatch if they are from a different brand or series, you could encounter volume level disparities even if the sensitivities are identical. But you can always level adjust using the receiver's calibration set-up.
I don't think your receiver would have any problem driving an 8-ohm load, unless the manual explicit says so. But I doubt that considering that 8-ohm is the standard for most receivers. In solid state amps, it is always a safer bet to go higher impedance speakers than lower ones, when in doubt.</font>