Adapters are designed to produce a maximum current, no more, but
certainly less. It's like a power supply. Just because it has 300W
of capacity doesn't mean it continuously provides that much power.
In all my computing years (about 7+), I've never had issues mixing
and matching adapters, as long as the things are compatible. Heck,
I use ASUS and Acer chargers interchangeably as and when I need to.
I do tend to buy oversized adapters for my friends when their
chargers kick the bucket and none of them have reported any
issues.
As for the issue you pointed out with Apple's MB/MBP Magsafe
chargers, official word from Apple is that they are
interchangeable, as long as the wattage is sufficient. Therefore,
the 45W/60W MB chargers will NOT work with the MBP, but the 85W MBP
chargers will work with the MB. (The newer 85W chargers have dual
voltages, one for MBP and one for MB usage. The chargers auto-sense
the required voltage. Unsure if the older chargers have this
function.)
That is the official word.
Yes they are interchangeable.
You can also use a MacBook Air/White charger for a MacBook or a
MacBook Pro.
Just that, if you use it for too long, it screws up the Mac.
45W & 60W chargers work on the MacBook Pro. My classmates
use my charger(MacBook) on their MacBook Pros all the time, but I
simply refuse to use theirs.
Older chargers do not have the auto-sense function.