You won't hurt the speakers, but you won't get out of them everything that you could. If a speaker is rated that high (keep in mind that each company makes those ratings in different ways), then it is probably designed for a high wattage system. The speaker will probably be built differently, with heavier components and stiffer surrounds, and have greater excursion as well. Basically designed for someone who wants to play it loud. What happens with your lower wattage amp? Nothing bad. You won't send it a distorted signal just by the wattage mismatch, that only happens if you overdirve and/or clip the signal before that point (such as having the volume maxed with the gains all the way up). You might possibly distort the sound from the speaker because there's not enough power to properly control the movement of the speaker, but I doubt you could measure it, if it even happened at all. Consider that a bigger amp that's capable of higher power will sometimes play music at a lower volume, and it's not like suddenly everything sounds horrible below a magic 50W threshold?

Also, it's been covered, but think of 12V as being the power supplied just from the battery (that is car turned off) and 14.4 is what is supplied from the alternator (car turned on). Various things can affect what the amp receives such as wire size, length, having your wipers or headlights turned on, etc. but it's not worth being concerned about. There's not a switch you can just hit to change from 12 to 14.4 or anything.

What exactly are the speakers you're looking at? There must be some others of similar quality, price, that sound good, and imply that they "need" less power.