Can I use 8 and 16ohm speakers in the same cabinet?

It isn’t advisable to mix speakers of different impedances in one cabinet. This is because power will not be shared evenly between the speakers and it can cause frequency dependent shifts in the power balance. This will sound terrible, it can affect the amplifier and may damage the speakers. It is always best to use speakers of the same nominal impedance in one cabinet. (Similarly, it’s also advisable to have extension cabs with speakers that have matching impedances).