I ask because the voltage is higher (16.5v vs 14.5v). I would assume the 60watt would charge faster, but not sure if the voltage increase will do damage.
Yes it is safe. but if you're buying, why not buy the 85W version to begin with? That one can be used with MBP's as well.
The power supply is not outputting constant amount of power at all times. The Watt rating only defines the UPPER limit of what the brick can do. The laptop defines the amount of power it DRAWS from the brick and if that is within the capabilities of the brick, then it's all good.
That said, you can also use a 45W power brick with MBP's, just that it's not providing enough power which means that either the battery is not charging (fast enough at least) or the brick itself might overheat. I wouldn't try my luck, but it's not that the brick would suddenly make the battery explode.
I agree with all you said, in regards to wattage, but what about voltage? If the MBA requires 14.5v and the 60 watt adapter is 16.5v, then I would think there could be an issue.