Re: Ohms setting??
FWIW, the formula for mismatched parallel resistance is R = 1/(1/R1 + 1/R2 + ... + 1/Rn) so in this case it is R = 1/(1/8 + 1/16) = 5.33
Mixing cabs of different impedances won't hurt anything as long the total impedance is slightly greater than the setting on the amp (for tube amps, for SS amps doesn't really matter as long as the impedance is above the min rating on the amp), but isn't really the best practice since it doesn't split the power equally.