It is not recommended to use a different impedance. If you use a 4 ohm high frequency driver with an 8 ohm crossover, the crossover point will double.
If you use a 4 ohm woofer with an 8 ohm crossover, the crossover point will halve. The result of using 4 ohm components with a 2-way 8 ohm crossover
 will leave a hole in the response. If you use a 16 ohm high frequency driver with an 8 ohm crossover, the crossover point will halve.
If you use a 16 ohm woofer with an 8 ohm crossover, the crossover point will double. The result of using 16 ohm components with a 2-way 8 ohm crossover
will be an excess of overlapping frequencies, and it could introduce abusive lower frequencies to the high frequency driver.