[BroadcastingEd]

1. Only a skeleton can spawn in the nether and in the overworld.
Well pigmen can spawn in the overworld. If a pig is struck by lightning it becomes a pigman.

4. Wolfs will attack skeletons because they are made of bones.
That's something about wolves not skeletons.

Here's one you might not know.
Skeletons can be left handed, there's a very small chance when a skeleton spawns it becomes left handed.

Also Pigmen can spawn from portals.