Ram, long term spoilt? if it doesn't work, probably it will not work in the 1st place. So it's a myth.

Should be no problem for the faster ram to run at 1 step slower speed. so the 667 should be ok.

The other concern is Dual Channel issue. Dual channel will only work if you have identical ram sticks. So if you have inside your lappy one 533 module,and you add another 667, you get bigger capacity, but maybe lose abit of speed advantage, but maybe not noticeable .