Using drivers of different impedances in a 3-way speaker

I'm building a 10" 3-way speaker, and I'm pretty green with electronics, so I need help with several questions. I'm using Morel drivers, and while the tweeter & midrange are 8 ohms, the woofer is 6.4 ohms. Is this a problem? Could it damage the amp? Could hooking up just the woofer to test it when the amp says "8-16 Ohms" damage the amp or speaker? Would I need any kind of special crossover, or can I just buy a standard off-the-shelf model?Many thanks for any help y'all can provide.
 
6.4 ohms is a very precise figure.
Are you building from a kit or a published design?
What kind of amplifier are you using?
What are the drivers?

You would likely need a combination of loud volume & low impedance to fry the amp.
 
1. ...while the tweeter & midrange are 8 ohms, the woofer is 6.4 ohms. Is this a problem?
No.
2. Could it damage the amp?
No.
3. Could hooking up just the woofer to test it when the amp says "8-16 Ohms" damage the amp or speaker?
No, but use very low signal if it is in the free air. Woofer must be in the appropriate enclosure to be tested at full signal from amplifier.
4.Would I need any kind of special crossover, or can I just buy a standard off-the-shelf model?
Yes, the crossover mast be tailored to those specific drivers you have. Do not waste your money on off-the-shelf crossover.