Yes, you can make objectives that must be completed to finish the level. But it sometimes requires some form of scripting so you'll find the sticky at the top of the scripting forum useful.
No, no teams or allies. The best you can manage is to have an enemy that doesn't shoot you, so therefore he is either a civilian or a hostage. You can't make allies that can kill other enemies...yet.
Search the showcase forum for it, there's a search function at the bottom of every page. As far as i know, i don't anyone has made that level.
AE