For the NeoPro 6.5s freq response is about 60hz to 4,000-5,000hz, they really drop off after 6,000 hz which anyways most tweeters are already playing in that frequancy. So if you have a tweeter such as a "Pro Audio" style super tweeter like say Seleniums that dont like to play to low in the midrange the neopro line will match up great with them.
The 60hz frequency is tested in an open freeair style application (as if it was in your door), If you build sealed fiberglass pods for the neopro drivers they will play a little lower and be safe doing it (but you must also take in the power your throwing at them). This line of speakers where made to be like a OEM door speaker where it is played in a open door that is not sealed, they will play much lower then your other types of "Pro Audio" speakers (again selenium, they lack midbass something fierce), not the case with the neopros when they are even used in a freeair application.
The 8inch neopros are asked to be crossed over at 70hz, both the 6.5s and 8s have a 2inch voice coil, same suspension and power handling, while the 10inch neopros use a 2.5inch voice coil and a slightly higher rms rating due to the larger coil.
Personally I would run the neopro speakers and whatever tweeters you chose to be ran active like others have said. So that means a 4channel amp would run 1 neopro per channel on 1 and 2, and 1 tweeter per channel on 3 and 4. Of coarse you could wire 2 speakers or tweeters per channel if you wanted to lower the impedance a little bit.
Here are some Neopro 8s and a pair of slenium st-400 tweeter I played around with powered by a sundown 100.4D ran active. IIRC I had channels 1 & 2 bridge to the mids at 4ohms (2ohms per channel), and the same for 3 & 4 powering the tweeters to get more power out of the amp instead of wiring the amp at 8ohms per channel since all speakers and tweeters are 8ohms. So the amp was doing ruffly 320watts per set of bridged channels, or 160watts rms per speaker. So you know the neopros are strong!
http://www.youtube.com/watch?v=vCwiuBZlLBU
Here is the response graphs from sundown for the neopros