As stockpiles are technically treated as buildings, stockpiles can not be laid over bridges.
No falling objects can hurt anything ever. Falling floors and walls are a different story, in that they kill everything in their way.
This is kind of a two part question, What do you feel is the best or fastest way to make legendary weapon or armorsmiths, work their skill piece by piece, or make a bunch of peasants into the appropriate smith and hope for a mood, or maybe some other way?
Also, how important do you feel it is for the dorfs to have a preference for whatever it is they make?
If you have a lot of ore, like if you come across several clusters of magnetite, you can usually afford to train your armorsmiths and weaponsmiths by piece by piece. If you have an armorsmith already at your fort, it will save time training him up than starting from a random peasant. Note that chain leggings and giant axe blades return half a bar back when you melt them, thus making them the best items to use for training up metalworkers.
Having a bunch of peasants make one piece of armor can also give you a legendary armorsmith fairly quickly, although sometimes you don't want to wait for one to get a mood.
Preferences are important, as they improve the quality level of items a dwarf makes if he works with materials he prefers or he makes an item he prefers. If he makes an item he prefers out of a material he prefers, the effects stack.