Nominal resistance is resistance to DC electricity
Impedance is basically fluid AC resistance as a woofer is playing. Consistent-as-possible impedance will result in a more balanced and stable system, overall.
You need to find a crossover that will handle what you’re trying to do and a powerful enough amp, or you should independently run your mids and highs on separate amps or channels.
Ohms are ohms, so nominal resistance in ohms is static resistance (without the woofers playing). Typically you never want lower than 2 ohms, and 4 ohms probably sounds better, overall. You could power those 3 door speakers per side with a massive 2 channel like said above by pop, and just get a small amp for the tweets.