I'm trying to set up a command block so when you press a button it summons a skeleton with a bow, wearing a red leather tunic, with fire resistance, named Rattle, and I want it to have 40 health rather than 20.
With the command following command everything works but it adds no health:
This also stops the mobs from attacking each other by putting them on the same team. If you don't want any of the mobs to attack each other you need to make sure they are all on the same team. Hope this helps.
Edit: Also to answer the question in your title, you can find fairly accurate documentation of every command on the Minecraft Wiki.