What does mean frequency range of a PA?

My Yamaha two-way 10 inch pair of speakers are designed to play a frequency range from 55Hz to 20kHz at +/-10 dB, 129SPL per speaker, measured in a distance of 1 meter from the speaker.

Questions:
If my speaker plays in 55Hz it has no difficulty to play above 80Hz will I have any troubles if I apply a 80Hz Hipass to them and send a 80Hz lowpass to a 18 inch subwoofer? If these speakers are designed to play from 55Hz, they are a good match for a 18 inch subwoofer, or do you think a 12 or 15 inch speaker is better?
 
Questions:
If my speaker plays in 55Hz it has no difficulty to play above 80Hz will I have any troubles if I apply a 80Hz Hipass to them and send a 80Hz lowpass to a 18 inch subwoofer?
No that is a typical and correct way to add a sub.


If these speakers are designed to play from 55Hz, they are a good match for a 18 inch subwoofer, or do you think a 12 or 15 inch speaker is better?
They are a good match. With a sub the system becomes 3-way so better to think of it in that way... no need for a larger low-mid driver in a system this size.
 
Your 10" woofer is actually a Low-Mid in the new 18" system.

"55 Hz" is actually "optimistic" Marketing babble, and the "within -10dB" confirms they are not actually happy getting that low, so 80Hz crossover sounds a good option.

If anything, I would even choose 100Hz or 120Hz, so the 10" woofer, which also handles a good proportion of mids, works cleaner and with more available headroom, and leave the ugly jobs to the 18" one.

I guess the 18" "should" be flat enough up to 100-120Hz!
Unless it´s a very narrow band high Q tuned boombox that is.