Specific examples always work better than painting with a broad brush. Here is an impedance curve (in red) of a Salk SongTower
http://www.salksound.com/songtower specifications.htm. When this trace was done, it had two 8 ohm CA15 woofers and one 8 ohm Hiquphon OWII tweeter:
You can see the effect of the two 8 ohm woofers wired in parallel with the 4 ohm minimum in 200-300 Hz range. Their impedance gradually rises as the frequency increases. The crossover frequency is about 2.5 kHz, and you can see the effect of the tweeter & crossover even below 1 kHz. Above 3 kHz, the tweeter's impedance levels off at around 6 or 7 ohms and climbs slowly higher as frequency rises.
To directly answer your question, yes you can use woofer(s) and tweeter with different impedance values. But those values vary with frequency – they are not a single constant number. That info is measured in the form of a frequency sweep and all those data points are entered into the crossover calculations.