Mixing 16 ohms speakers in a 8 ohms speaker cab?

Im guessing your cab has 4x 8ohm speakers wired as 2 parallel pairs:

each pair=1/8 + 1/8 =1/4 so 4ohms x2 =8 ohms

if you replace one pair with 16 ohms it would be 4 ohms + 8 ohms = 12 ohms overall. It would be safe to run but you would get less output (might be a good thing?)
 
I once replaced two 8ohm speakers with 16ohm speakers on a Marshall 1960a. Worked just fine but I didn't want to risk anything breaking so I got new speakers to replace the 16ohm ones and sold the old ones to our other guitarist (And he hasn't paid for them yet. We'll see if he ever will or do I have to take them back).
 
I've never tried it myself, but it should result in a 3dB volume difference between the 16 ohm and 8 ohm speakers, with the louder ones depending on the wiring.