If you do go with tweeters then the usual crossover point is no higher than where the woofer 30 degree off-axis response drops to -6dB.

The sensitivity differential question can be addressed both by the wiring scheme, using different impedances of the woofer and tweeter lines to better match sensitivity, and with the crossover. With an 8 ohm woofer line, for instance, you can wire the tweets to 4 ohms or 16 ohms to raise or lower the relative tweeter sensitivity by 3dB for better matching.

It's a good idea to have the tweeter and woofer line heights reasonably close to each other, so that they will have the same radiation characteristics at the crossover frequency.