1. I have a ton of xxx skeletons or xxx partial skeletons, yet I can't do bone greaves with them. How to use these skeletons? How to make bone greaves?
If the skeletons are from intelligent creatures (goblins, elves, dwarves, kobolds, etc) or tame animals that belonged to your fortress, they are useless. You can't butcher intelligent creatures, and this includes breaking up their skeletons for bones. The same goes for tame animals which died from any cause other than being deliberately slaughtered at a butcher's shop.
Damn, so all these skeletons can't be used...
from the wiki, bones greaves are considered plate armor, so they are better than copper right?
I'm not sure if bone type matters to a dwarf having a mood, so keeping intelligent humanoid bones in a stockpile right next to a craftdwarf workshop might be handy. Never tried it. Not sure if you can set a stockpile this precisely. You surely don't want partial skeletons or parts stored there, for miasma risk. Just bones.
As for pet bones, those can be handled with coffins. Build a few coffins and allow the dwarves to use them for pets, and pet remains, dwarf remains, and friendly visitor remains should go to the cofins, 1 coffin per entity.
No skeleton can ever be used for anything, far as I know - it needs to rot into a pile of bones first.
Storing miasma-creating remains (stuff which might still have meat or soft remains on it) outdoors in piles prevents miasma, outdoor remains don't make purple stink, they just rot.
Putting workshops that might create refuse outdoors is a problem, unless you have outdoor gathering enabled.
If you have outdoor gathering enabled, that can cause problems with your dwarves going nuts when caravans are butchered by sieges or ambushes.
Best practice is probably to have a strict entrance/exit control to your fort including an outdoor area for grazing, farming, refuse, whatever, and disable outdoor collection of refuse whenever you open the gates.
An airlock style meeting area for immigrants or even an airlock style false-depot for traders might be a good plan for even better outdoor dwarf control.